This guide involves renaming the save file, if you decide to delete the save file instead of renaming it, ALL your progress in that area will be lost and you will have to to redo everything.

Also no need to disable Steam Cloud with this guide. Make a backup at your PC if you want.

1. Go to another area close to the area where the missing person should be.
2. Save it.
3. Don't end the game but go to desktop (Windows).
4. Go to %UserProfile%\Saved Games\Arkane Studios\Prey\SaveGames\
5. Select the right campaign folder and go to TEMP.
6. Rename the .level where the missing person should appear (rename it to anything).
7. Now go back to the game and load the savegame you did created in step 2.
8. Enter the area where the missing person should appear.
9. Be aware, the area is now full with many enemies but your missing person should also be there.
10. When you finally find him or her, go back to the area where you were in the beginning.
11. Save in this area.
12. Don't end the game but go back to desktop (Windows).
13. Revert back your changes (delete the new level file and restore the old level file)
14. Go back to game and your missing person is still marked as found and your area is back in the state before you did all this steps.

When ending or restarting the game during the steps, the game will repair the files with the original files, you really need let the game run in all steps otherwise it wont work.

TIP: make a shortcut to %UserProfile%\Saved Games\Arkane Studios\Prey\SaveGames on your desktop. It will save time when you run into problems again.

In this example i have renamed lobby.level to xlobby.level and the game should create a new "fresh" lobby.level.

When you return to the other area you need to delete the new lobby.level and rename xlobby.level to lobby.level.

arboretum.level = Arboretum
bridge.level = Bridge
corporateit.level = Crew Quarters
exterior.level = Outside Talos1
lifesupport.level = Lifesupport
lobby.level = Lobby
powersource.level = Reactor
prototype.level = Hardwarelabs (Confirmed)
psychotronics.level = Psychotronic
shuttlebay.level = Shuttle bay
simulationlabs.level = Where Morgan first time wake up.
zerog_utilitytunnels = Zero-G tunnel

This guide is great when Kirk Remmers is missing (should be in Life Support) and you cant complete the side mission, or when 1 of the 4 "players" from Treasure Hunt is missing. Or you want to find everyone for the achievement "missing persons".

Thanks for reading and hope this guide helped you!