In the "showers" chapter, right after Riddick gets his eyeshine, stealth kill the guards near the lockers without letting them activate the alarm. Once that's over with, open the lockers and you'll find some UD, assault rifles, a pack of smokes, and a guard suit. Put on the suit and you can walk around without having to kill anyone (but you can't go into stealth mode, or kill any lights, otherwise they'll figure out you're a prisoner). You can talk to the guards, giving you some valuable information when you reach the "Guards' Quarters" chapter and try to find Abbot. At the place where Riddick uses the retina scanner, go to the end of the room and to your left (the side where you first come through) will be a ventilation shaft. Enter and you'll find yourself in a small room with assault rifles and a light armor guard suit. Use the
armor, and you can still act as a guard. This secret allows you to obtain some extra UD, and makes everything much easier when you try to reach Abbot.