After you find the camera, in a level 6 security door behind a destructable wall in armory south, take pictures of random stuff. When viewing the pictures, you will see other pictures of some freaky stuff laid over top of yours, and there will be a new option that says "exorcise". All this does is makes the ghost go away, so do not do this if you like the way it looks.

Verified Ghost Photos:

1. The Dead DARPA chief (in the medical room holding cell, not Decoy Octopus)

2.The bust with the bandages on it's face on the left side of the Command Room (where you fight Psycho Mantis)

3. The mirror on the right side of the command room (where you fight Psycho Mantis)