Originally posted by Andros
The thief won't show up unless you turn on the power and I'm pretty sure you have to be downstairs (as in not in the starting room) for him to spawn.
Anyway, the easiest way to get the trophy is to upgrade your Colt 1911 and wait on the railing for him to spawn. As soon as you see the red myst (whatever you call it) start bombing it with your upgraded colts and he will go down in a few hits. You don't need for him to be in "human" shape to kill him.
You CAN be in the starting room and have him show, as long as the power is on (As he uses the teleporters). We tried avoiding him by going up and down in the elevators when he got close, but it won't let you even close the doors when you can (Or anyone else) visually see him.
-Result: He took all of our guns x)