This guide is meant to answer the question "How many sentries should I take to mission without leaving any behind?" It contains both spoiler free summary with number of sentries available only and detailed guide where to find each sentry on the map.

This guide also counts with doing the mission with minimal amount of deployments. If you need more, just substract the number of found sentries in your previous deployment.

Number of sentries: 2

1. 1st Floor, In the Veterinary Clinic, part of saving veterinarian McNeil mission
   
   
2. Mineshaft -1, west side of queens nest
   

Number of sentries: 1

1. 2nd Floor, NE corner in the Control Room (camera icon is overlaping it on the map)
   

Number of sentries: 4

1. 1st Floor, SE from the starting position, to east from second ARC point
   
2. Basement, the northernmost chamber with spawning point
   
3. Basement, chamber with the final battle (2 sentries at once)
   

note: after the second wave with Crusher, the mission with end automatically, there is no manual extraction - make sure you pick up your sentries after first wave

Number of sentries: 1

1. Otago Crash Site, eastern part of the map
   

Number of sentries: 3

1. Headquaters, all 3 are in the Armory Room in the center of the map. You will get send there after you get the Codebreaker as part of the mission - you cannot get the sentries before you progress with the story
   

Number of sentries: 5

1. 1st Floor, in the Laboratory building behind destructible walls next to Lisa.
   
2. 1st Floor, southern Hangar
   
3. Basement 1, in the Generator Room
   
4. Basement 1, behind locked door SW from Generator Room
   
5. Basement 1, north from Generator Room
   

Note: you need at least two deployments for this mission as finding the APC will end the current deployment. Only first four sentries are available before you have to end the first deployment, so you can ratio the sentry pick up

Number of sentries: 5

1. Deck B, in the Electrical Room, SW from the elevator next to the middle sleeping pack of aliens
   
2. Deck B, west from the elevator behind locked doors
   
3. Deck B, in the Maintenance Room, south from the room with the Loader
   
4. Deck B, in the Armory Room in the NE
   
5. Deck B, in the Land Survey Preparation Room in the SW
   

Note: The start of this mission is happening on the same Berkeley's Docks map as mission 2, so if you missed sentry on the 1st Floor, you can collect it now. The Basement level is unaccessible though so if you missed any of the three sentries there, they are gone for good.

Number of sentries: 2

1. 1st Floor, top part of the Hangar
   
2. Basement -1, to the south from the room with cryopods
   

Number of Sentries: 0

Number of sentries: 3

1. Office Floor, NE corner behind the last Commando team
   
2. Security Floor, room with destroyable wall in the west (2 sentries at once)
   

Number of sentries: 5

1. Basement 1, to the south from Camera Room
   
2. Basement 1, to NW just outside the Control Room
   
3. Basement 2, to SW in the Armory
   
4. Basement 2, behind destructable walls to the west
   
5. Basement 2, to the east in the Generator Room
   

Note: this is the final mission with marines, you are not returning to Otago once you finish this mission

Number of Sentries: 0

That's all. If you find any mistake or anything else, let me know in the comments.