This trophy is earned for finding optional photo #5. 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:
[
2:55] - Optional Photo #5
After breaking into the principal's office, take a picture of Chloe sitting in the office chair from in front of the desk.
Collecting all 10 photos will also reward you with the following achievement:
https://www.trueachievements.com/a197694/episode-3-secr....htm