Proceeding through the game, you will be forced to find all but four pieces of evidence (See 'Gold Game Completion' for help if you are stuck). The "hidden" evidence is hard to miss, but here it is:
Chapter 2: Investigate the button on the elevator in the basement for a fingerprint.
Chapter 2: Before you move the locker at the end of the level, check the door for scratches.
Chapter 4: Extremely hard to miss, but by the end of the level, you have to check the map twice. Once a wide-view picture (required), the second a close-up on the circled area ("hidden").
Chapter 8: On the left wall of the first corridor for a stain of blood.