On the level where you get the companion cube after you run past the energy 
balls and are in the room with the three elevators, there are two doors leading 
to a catcher and a platform that is pushed out of the wall. Walk up to the 
platform that is pushed out. Crouch behind it, and walk through the opening. On 
the left hand side are a group of pictures of people with boxes for heads and 
hearts around them.