This trophy is earned for finding optional photo #4. If missed on the first run, you can "Play Episodes" and select specific parts of the Episode in order to find the missing optional photos through collectibles mode. The following collectible is described and can be seen below:
] - Optional Photo #4
In the school, in the science room, there's a skeleton at the back of the room with a cigarette in its mouth.
Collecting all 10 photos will also reward you with the following achievement:https://www.trueachievements.com/a197694/episode-3-secr....htm