You may have already figured this bit out: Prey has hidden fire alarm safes. Not every fire alarm is just a normal fire alarm, though. Hit them three times with your wrench, and they might open up to reveal a hidden safe.
But, before you go around hitting every fire alarm you see in hopes that it's a hidden safe, there is a foolproof method of spotting the ones that contain goodies.
If the fire alarm has a red light above it, it's a hidden safe. If not, then it's a normal fire alarm.
Keep your eyes peeled for these hidden treasures and you'll find yourself rewarded with valuable loot.