Community guide to cut down the time it'll take to find all of the hidden cache locations. Help me locate them all.

If you find a hidden cache in a map that hasn't already been found, please give a description of where it was located in the comments. Once there are 4 or more locations for a map I'll go through that map and try to confirm them and possibly add a screenshot of the room.

Physical description
Hidden caches are flat orange crates with two bright yellow lights on each bottom corner. They are the same standard size as the grey consumable crates that are located at swarm defense points where you find ammo crates and med kits together.
A black and white crate symbol appears above them, this can be viewed at a considerable distance as long as the crate is in line of sight.

What do they contain?
They each generally contain three different items, generally a weapon attachment(unless you have them all), a cosmetic, an emote, consumables and challenge cards. Rarely you can get a 4th drop from a cache which include brand new perks e.g. "Creative Pain Point Solutions" for Tech. They are the fastest way to get new attachments besides completing missions with "Form An Attachment" challenge card and the selection you can buy directly from the shop. You will always get a unique item when possible, not something you already own, with the exception being consumables.

Mechanics
One cache will spawn in every level in a random location. The locations it can spawn in are fixed, so knowing where to look and then not looking after you find one will speed up game play considerably. There are approximately 8 different spawn locations per level (to be confirmed).
Opening 50 hidden caches will award you with an achievement called Keen Eye.
You will keep the loot even if you die or abandon the mission, so they have potential to reduce grind. Difficulty does not seem to increase the loot in cache the same way it affects end of mission rewards, although its random.

How to find them?
The ones I've found so far can typically be found by looking behind you as you progress towards the marker. There are a few exceptions where they are located off towards the side. They are generally visible without going off the path if you happen to look in the right direction, again there are some exceptions and these might just be the easiest ones to locate. If you find a new one, please write in the comments so I can add it to the list and narrow down possibilities.

Hidden caches produce a loud audible cue, that is louder than most background noises. It can be difficult to near-impossible to discern this unless you know what to listen out for. You can't hear it during combat or speech.
The first noise it makes is a low and constant drumming whirr. This is overlapped with a fluctuating higher pitched whine. Lastly, when you are very close you can hear the monitor on top make a printing noise. Concentrate on learning the first two noises as they are quite loud and can be heard from some distance. This can help you locate caches in previously unknown locations.

Sometimes, hidden caches are very well hidden, popping up behind random tents or in caves on dead-end paths. They can spawn near Intel sometimes.

All caches will be listed in the order you can find them where possible. Requires knowing all of the locations for a level and an accurate description of where to find them.

Priority One: Ingress
1.1.1 Opposite the first door you are told to cut through, behind you, behind the blue box.

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1.1.2 "Unlock Exits", 1st working Joes area, second(far) back left room, in opposite corner.

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1.1.3 "Prepare for Battle", down far stairs, near facehugger, head towards "giant Dalek head".

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1.1.4 3-way ambush (after drone ambush), inside working Joes area, on the left.
1.1.5 3-way ambush (after drone ambush), inside working Joes area, behind the flatbed on the right.

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1.1.6 Ammo box ambush (prowlers, bursters), downstairs, far left corner past Intel(confirmed, thanks reaker).
1.1.7 Ammo box ambush (prowlers, bursters), upstairs, turn around at exit doorway before you leave for the last time (2nd time if you get the random side mission). Behind some boxes.

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1.1.8 Huge coolant room with internal leaks and safety rails. Turn left as you enter(chance of prowler).

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1.1.9 After coolant room, past bursters and %prowler. On the left, enemies may spawn from there.
Last chance to grab it as the next door seals you into the final defense area.

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Final defense notes: Its possible to wipe out the aliens with at least 20 seconds to spare, then nothing will spawn for 20 seconds. The final wave only spawns when the elevator timer hits 0.
Its possible to place a turret on the far right of the box behind the rail on the right (just past the spawn tunnel), where enemies and Warriors can't reach, but they'll still try to attack it.


Priority One: Rescue
The defense area ahead has a quiet computerised background hum of its own.
1.2.1 In the first defense area, up the stairs on the left as you enter this area. Look right and it will be up against wall in the opposite corner, between the catwalk and an inaccessible staircase that leads up to the defense objective (used by xenomorphs during the event to outflank the team).

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1.2.2 "Override Door Lockdown", defense area, up the stairs on the left as you enter this area. In a corner behind consoles that will be in front of you as you reach the top of those stairs.

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1.2.3 "Override Door Lockdown", defense area, up the stairs on the right as you enter this area. Look left and it will be up against wall in the opposite corner.

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The area you search for the valve is quiet, so you will hear the cache easily.
1.2.4 "Locate Valve", take a left twice, the tiny storage room on the left and the first valve room to check. It will be on the right wall (%prowler).
1.2.4b "Locate Valve", alternately it may be opposite the %prowler spawn instead, so same room but on the left side.
1.2.5 "Locate Valve", same area, but the next room to search along, on the left.

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1.2.6 The "shooting gallery" of pipes and barricades just before the room where you override the inner door locks (and meet Monica). Mission control will announce "They're coming through the pipes!" and then you'll have to fight off several waves. After finishing off the enemies, move forward until you reach the exit. Check the side tunnels on the left and right for a possible spawn.

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The area you first encounter Monica has a quiet computerised background hum of its own.
1.2.7 "after Unlock Inner Door" After 1st Monica encounter, from ammo box, go down, take first right in crate maze and its on the left.
1.2.8 Same area, from ammo box, go down and straight ahead, in front of a spawn vent, behind large trapezoidal dumpster crate.
1.2.9 Same area, turn left at the crate just mentioned onto to a dead-end balcony and that's where it is.

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1.2.10 Same area, when about to leave(Bashi Tower Centcomp A Deck 41), instead of going into red lit area, turn left to stay in the same area, then turn right and its next to a closed door.

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Bonus: Notes on Monica
After the Prowler on the right spawns, she will run down the middle from where you came from. You can mine this pathway, although on harder difficulties Warriors will come from this direction as well.


Priority One: Extract
1.3.1 As you exit waste disposal/sewers, turn right instead of left. Across this bridge (opposite waypoint marker), under 2m crate with an X and next to blue barrel.

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1.3.2 Next area(inside). Take a right instead of the waypoint door 13m ahead, then another right into dead-end.

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1.3.3 "Prepare for Battle", go down the stairs on the right and check around the corner, before the red meters, next to the two piles of pipes.

There's a bug here. Placing a turret in front of the crate barricade next to wall can trigger the next area to spawn enemies and dialogue.

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1,3,4 Same area, check to the right of the door before leaving.(unconfirmed?)

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1.3.5 After you check synths for a door code "download synth data" and bypass the door. Instead of going left and down into the pool, go to the end and look inside the doorway to the right and it will be filling the doorway.

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1.3.6 Beltway area. Directly below the beltway, beltway-side/red meters in the middle.
~2nd Intel.

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1.3.7 "Investigate Hangar Delta", take a 180 left before the loading arm and go up the stairs behind you up to the tower control room door(closed).

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1.3.8 Next area, long corridor with red meters on the right. At 30m to waypoint door, take the dim side corridor on the right, about half-way down.

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Giants in the Earth: Insertion
2.1.1 In the large early cavern, turn fully 90 degrees right from the ammobox, it should be in front of you against the wall.
(PLACEHOLDER FOR SCREENSHOT AFTER ITS COMPRESSED)

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2.1.2 Church-like area with large pipes upperleft and right and radioactive spitter. Go to the very back past the aisles, its hidden around the left of that wall.

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2.1.3 Immediately after that area, take the side corridor on the left. It's before the "bug swarm" battle and 2nd Intel. Also a strong fallback position.
(PLACEHOLDER FOR SCREENSHOT AFTER ITS COMPRESSED)

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2.1.4 Circular pit with a cable overhanging through the middle. Clearly visible inside the pit from the top.(confirmed)
(PLACEHOLDER FOR SCREENSHOT AFTER ITS COMPRESSED)

2.1.5 Same pit. Go down and its behind a stone block on the right.(unconfirmed)

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2.1.6 Same area as pit?, turn away from waypoint marker, turn right and go up the ramp before the crumbled wall and its on the right. (need to recheck location)

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2.1.7 20m before the door at the end of the long cable corridor, before you go outside. Turn left. Point of no return here.

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2.1.8 "Access Bridge Controls". Behind the left pillar at the back.(unconfirmed)
(PLACEHOLDER FOR SCREENSHOT AFTER ITS COMPRESSED)

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Giants in the Earth: Contact
2.2.1 In a tunnel after overgrown section, which is the next tunnel after the giant head. At about 45m from the next waypoint, the second right in that tunnel underneath bright blue external light from above, behind a wall (the first right is caved in).
(PLACEHOLDER FOR SCREENSHOT AFTER ITS COMPRESSED)

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2.2.2 "Prepare for Battle", behind the first wall. (On the right if you turn around, on left as you enter)
2.2.3 "Prepare for Battle", further along but against the right side of this area instead, hidden behind a bush.

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2.2.4 "Synth vs Xenos" Open area with many low blocks, at 30m to the exit on the right, turn left and itll be in the left corner of that "balcony".(confirmed)

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2.2.5 Large open room, turn left and its against the left wall.

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Giants in the Earth: Evacuate
2.3.1 At the very first battle with synth & xenos, around 30m to the exit, go down the stairs in the middle under the exit. It'll be on the right, clearly visible.(confirmed)

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2.3.2 Dripping room with giant faces and a well in middle. Turn right (opposite waypoint marker) and go up the low stairs.

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2.3.3 "Collect and Attach Synth Core"/1st Intel. Go up the stairs next to the exit and turn left, its against the balcony wall.

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2.3.4 Next area or possibly minefield area, large centre well with tattered red banners on pillars. ?
Go up the left stairs and its against the wall (around 52m from waypoint).(check)

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2.3.5 Minefield area. (check) At around 70m from waypoint marker(right) turn left and its in the left corner.?(check)

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2.3.6 Circular area with tall pillars with banners. Go up the left stairs and its against the wall (around 52m from waypoint).(check, duplicate?)

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2.3.7 Ammo box with tree roots above a filled well/dais. Take the stairs left and behind you, follow them up to the right and its in a dark corner. Watch out for mines(confirmed)

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2.3.8 Working Joes trap room. In the lower area on the left before the stairs going up.(confirmed)
2.3.9 Working Joes trap room. In the lower area on the right before the stairs going up.

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2.3.10 "Cynthia reveal." Two dais overlooking an endless drop, turn left instead of right to 40m objective and its against the right wall before the slope upwards.
2.3.11 Same location, but its at the top of the slope against the right wall.

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Final battle notes:
Kill overcharged wardens(green dots) to get power cores. You can complete the battle quickly if you rush the 4 cores required. Snipers can KO bot allies on Intense+, they're worth killing.

The Gift of Fire: Recon
3.1.1 "Investigate Holograms" In the first large area (furnace,dead synth), take a left and its on the right next to a blue cylinder light, opposite the exit to this area.
?3.1.2 "Prepare for Battle/LIDAR", behind the far pillar before you cross the bridge.?
3.1.3 "Prepare for Battle/LIDAR", cross the bridge and it'll be on the immediate left, bridge corner.
3.1.4 "Prepare for Battle/LIDAR", cross the bridge and it'll be further ahead past the blue light, against left wall(unconfirmed).
3.1.5 "Investigate Anomaly", at the little "ufo"/blank control panel + ammobox, turn right and go up the stairs behind you, its at the top of the stairs.
3.1.6 "Prepare for Battle", after the hologram room. Take the right "intestinal tract" and you'll pass right in front of it on the way up before the room opens up to the defense area.

The Gift of Fire: Advance
3.2.1 "we need a map/these halls are nuts/radar is worthless" Long open corridor, with 2 walkways, mantle onto the higher walkway and its on the left next to some tripods next to 1st Intel(easy to miss).
Notes: The three Instrumentality on this level may make a constant loud humming noise similar to caches, but without the constant low sound effect.
3.2.2 "Deploy Scanner". It's on the lower level against one of the low walls.(pending more accurate description)
3.2.3. after second Instrumentality. Head right of objective and take the small ramp up to 2nd Intel, a cache may spawn here which will be visible as soon as you enter.(confirmed)
3.2.4 in the large room after that, it'll be in front and in the back of the area a little below you as you enter. Should be visible once you've moved forward.

The Gift of Fire: Boarding
Notes: Like the previous level, this level has a constant background hum whenever near Engineer technology that will make it harder to tell whether or not you're near a hidden cache.
3.3.1 Hologram of spaceship room. Go right around the hologram and its in that corner of the hologram table.

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3.3.2 Immediately after, there are seated Engineers on the right with the 1st Intel. Instead of going through the door, turn left and its on the right next to the console with pale green buttons.

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3.3.3 After the circular room. T-junction with oval ends in front of you, turn left instead of going up right to waypoint. It'll be clearly visible & audible on the left.

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Notes on final battle:
The enemies mainly come from either side, not down the middle. They will eventually stop coming even if you don't destroy the first capacitor, until you destroy it. Micro Rockets deal no damage to the capacitors, you need to shoot the three glowing red weakspots out. They take e.g. six bursts of the AM-16 Gruppa rifle or two M12 RPGs.

The Only Way To Be Sure: Breach
4.1.1 "Hack Door Systems". At 35m from waypoint, it'll be on the left near the top, next to cylinders, barrels and a rail stand. Easily visible & audible as you pass right by it.

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4.1.2 After the long stairway, you'll come to a small ambush room with two levels. The cache will be inside a doorway, on the immediate right as you enter.

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4.1.3 Large, triple sentry room. All the way at the back, next to left stairs.(1st Intel area)

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4.1.4 When you encounter alien eggs, keep going right(3 times) past the yellow loading arm above, its behind the 2m+ loading crate.

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Notes on final battle: Its split into two opposite fields, so you can save some deployables for the second half on the opposite side.


The Only Way To Be Sure: Search
4.2.1 At mission start in a large area with some kind of secreted resin (webbed ceiling). Backpedal to 47m from waypoint, turn left and its in a closed doorway, against left wall.
4.2.2 In the tunnel after exiting first room, at the flare 28m from waypoint, turn right, and its on the right.
4.2.3 Same area. At the flare leading to door ~17m, turn left, run past the boxes and its at the end.(by 1st Intel)
Notes: The background echo in this area drowns out the noise of the cache, but you can hear the monitor printing in close proximity. After 2nd Intel.
4.2.4 "Take Dogtags" From the ammobox, look left and its directly in front.
To check:
?4.2.5/4.2.7? "Prepare for Battle", go to the far end and up some stairs on the right.
?4.2.3b/4.2.6? Go down the long flight of stairs, its in a room on the left.
To check:
4.2.6 Past the ammobox, at the top of stairs to the right of waypoint marker next to a closed door.
4.2.7 From the ammobox, run towards the waypoint marker, its in the corner.
Notes on final battle: Look up


The Only Way To Be Sure: Regicide
4.3.1 After you cut through the first door. Head along the short tunnel past the next door. Head straight ahead instead of turning right towards the waypoint and it'll be on the left.
4.3.2 After the elevator, in the cocoon room, it'll be behind the pipe on the right as you go past.
Notes on final battle: The finale lasts a long time, save your consumables for that battle. The battle lasts presumably longer than the default timer on sentries meaning each player can save 1-2 sentries to place after you finish the first sequence.

1. Change from Windowed Fullscreen to Fullscreen (Fullscreen itself can cause crash problems, see below)
or
2. Add "-LockMouse" to launch options. Including the quotation marks.
I've been running this in launch options along with playing in Windowed FullScreen to avoid crashes and haven't noticed any problems for a while.

The game produces full crash logs and stores them in the Appdata folder each time the game crashes. These folders can be over 8gb each. See folder: Main Drive/Users/(profile)/Appdata/Local/Endeavor/Saved/Crashes

DEVELOPER - The game crashes whenever the game is 'deselected' or alt-tabbed while loading the Endeavor or into a mission and this produces one of these logs, having the logs is one thing but the fact it is producing the whole log and storing it could completely crash and bugger up peoples computers (from my knowledge), this should be rectified and compressed/only key files saved.

Circumstances that can cause crash while playing in fullscreen and whenever the game is on a loading screen include the mouse being no longer 'bound' to the window, causing a crash if the game thinks you have clicked away from the screen or if one actually presses "alt-tab". The games loading circle will freeze, the game itself will freeze and slow down the system including how long it takes to open Task Manager. A UE4 crash window will appear and a crash report will be generated on C/main drive in above location. The longer you have been playing the larger the crash log but they can start off large or after just a single mission if you crash going back to the Endeavour.

There are 6 of every kind of attachment.

Armature: Quick Charge Armature
+25% Reload Speed
+15% Fire Rate, +30% Stability on Hit for 5 seconds (assuming this part isn't bugged, having both fire rate and reload speed is really good)

Armature: Deep Railed Armature
+30% Reload Speed
+20% Max Ammo (when you just want more ammo)

Barrel: Miller Twist Rifling
+20% Stability
+20% Handling
20% chance on Hit: Enemies are Slowed by 50% for 10 seconds
Useful on any weapon with a high fire rate and/or area of effect, noticeably the Smart Gun. Applies a visible debuff to the enemy. Combine with high Stumble or knockback to keep an enemy at bay.

Magazine:
Specials effects may be bugged. Needs further testing.
There is a medium magazine called Smart Rounds which gives +20% Aim Assist. There was a thread about it, here:
https://gtm.steamproxy.vip/app/1549970/discussions/0/3032599335601155806/
So you can decide whether or not they're worth using.
There is an internal magazine that has a minor chance to leave pools of fire that deal a static amount of damage.

Muzzle:
Specials effects may be bugged. Needs further testing.
Thermal explosion is only 2.5% damage, may be insignificant. Might have some potential on sniper rifles.

Optics:
Specials effects may be bugged. Needs further testing.
There is a large radar scope that gives you a similar effect as Recon's PUP while looking through the scope, as it detects enemies behind walls including prowlers.

Just some bonus stuff for finding the hidden cache in the guide.
Requisition credits cap at 50K. You can always buy consumables e.g. turrets.
Reputation scrip caps at 10K. You can always buy challenge cards.

There are 39 pieces of intel. They are not all ordered by mission or order of collection which can throw you off if you're missing some.

There are 4 outfits for every class plus a horde slayer outfit.
No idea how many cosmetics and weapon colors there are, let's say a lot.
There are at least 40 weapon decals.

There are 61 different Challenge cards. XP & credit rewards scale to difficulty e.g. damage dealt.
You might as well use up "Form An Attachment" before you max out attachments but if someone plays an XP/credits card with no downside, let them.

There are 11 different consumables. My favourites are hardened sentry guns.
There are 6 attachments of every type, which is mentioned in the previous section.
There are 9 rifles, 8 cqw(9 including heirloom), 7 handguns and 6 heavy weapons for a total of 30 weapons(31 including heirloom).


Horde Mode
The cabinet to spend your horde points is in front of you and to the left, on the left side, in the corner when you enter the main area. You get a reward for every 10th wave you pass.
Rewards from horde mode don't seem to scale with difficulty very well, meaning expect awful rewards if you play on higher difficulties.
As an example, Insane mode rewards you with around 155 Rep Scrip and 2250 Credits.
Also it will tell you what rewards to expect if you continue, but its a little misleading as it can show you perks that you already own, meaning you won't get a reward personally.
However there is a unique reward for beating wave 10 the first time on Intense, Extreme and Insane difficulty. These rewards are body/head outfits or a weapon reskin called Horde Slayer.

Conversely, rewards from campaign scale with difficulty very well. Use those high XP/credit challenge cards on higher difficulties if you have no problem with them. - Although if you're pubbing, challenge cards that make it easier may be appreciated too if the other players aren't using XP/credit cards of their own. Especially on "Win on Intense/Extreme" dailies, as the same dailies are shared by everyone.


What does each stat do?
Go to Equipment:Loadout and click on a weapon and you can mouseover each stat including the hidden handling stat but essentially:
Handling = also affects accuracy penalty for moving, decreases equip/ready time and time to enter ADS
Accuracy = affects spread range(min-max), and how quickly spread increases by firing or moving
Stability = expected recoil (inverse)
Stumble = %chance/shot to interrupt and stumble the target (higher=better). My listing = Stumble,Stumble/s(based on fire rate+pellets)
WeakSpot = typically the Head, except on Heavy Flamer Synth
Stamina is used for dodge rolling. You can normally roll three times in a row. Rolling gives you temporary invulnerability frames. Spacebar(default) is also used to mantle onto short ledges.


Attachments
Several single shot weapons have a noticeably high fire rate that won't be much improved by fire rate mods. Some go as high as 5.0 or higher. Your mileage may vary, so test with Gunner's Overclock and find out how fast you can shoot to check whether there's a difference.


Consumables
Sentry turrets(except Techs personal turret) eventually break down but usually last long enough for a defense wave or until an Elite destroys them on higher difficulties. This means that you don't want to set any fallback turrets on the final campaign mission as they won't stay there. Save them all for the big finale defense. It also means they won't last long during Horde mode, but nothing does.


Enemies:
Prowler stalkers(when stationary, no dot) emit heavy hissing that is usually audible well in advance unless the announcer is talking. They can be detected by PUPS or the large radar scope attachment. If you place a turret before alerting them, and fallback after, they tend to waste their attacks on the turret. However Prowlers are generally easy to deal with as long as you get the jump on them instead of vice versa.
Drones and Elite aliens(red dots) announce their first arrival with a scream. The announcer can call them out too. Elites tend to randomly target players or turrets, they can destroy turrets in one hit on the higher difficulties. Elites can perform instantly lethal grapples(no revive) on Insane.
Drones(green dots) will perform hit and run attacks. They will focus one player and try to grapple them unless they dodge roll, with their attack weakening with each failure(similar to prowler). They will continue until they land a hit or lose too much health, then perform their retreat through a nearby vent. If you get caught in their retreat you get trampled but take no damage. Drones move incredibly fast and are barely affected by Slow debuffs if at all. After a Drone hides it'll reappear a short while later without much warning, minus whatever health it lost.
Elites (at least Warriors) are greatly affected by Slow & Stun/Stumble debuffs, if you tag one directly with a Charge Coil or keep them under sustained Debuff or Knockback they can be held back.
Elites with skull crests e.g. Crusher & Praetorian, do not take any damage if you shoot them in the crest area. There is a Synth enemy with a riot shield that works the same way.

Hit markers:
Red = normal hit.
Yellow = weakspot hit.
Grey = no damage.
Large red X = killshot.
Trumpet sfx = weakspot killshot.


Insane difficulty perks & abilities:
There's one perk which scales directly to difficulty which is Demolisher's Fire & Forget size 3 rockets perk. Along with taking 25% more Area damage, they also bleed for 8% of their TOTAL life over 8s.
Gunner's Overclock ability can be essential for bringing down the health of major enemies (green & red).

Go debrief with the crew in the Endeavour.

This question has been showing up in one form or another on every page in Steam and its always because you need to talk to the crew. So I put the answer here.

Some perks might be so busted they do the opposite of what they say, while others may be working as intended.
https://gtm.steamproxy.vip/app/1549970/discussions/0/3033725780706582982/
Requires significantly more testing and more transparency in the game and in tests.

If I catch more threads about potential bugs I'll post them up.

Everyone
1.1.6 confirmed thanks reaker.
3 more spots on Priority: Rescue, thanks Dent.

I'll fill in the first 24 locations for Priority One and we'll go from there.