In Prey, you'll find the Armory in Psychotronics, with the door being next to a prisoner. You can get into the Armory with Mimic Matter by fitting through the slot in the hallway, but, if you haven't unlocked that skill, yet, you'll need the door code.
The code for this door is 8714.
You find this door code by releasing the prisoner with the computer in front of his cell.
Prey (2017) Armory Door Code - GameRevolutionWATCH GALLERY