This achievement is earned for finding optional photo #9. 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 #9
Once inside the stealth nightmare with the locker, you'll need to sneak past the first to enemies and hook around to the right in order to find Warren's locker. Open in and photograph the inside.
Collecting all 10 photos will also reward you with the following achievement: