This achievement is earned for finding optional graffiti #6. If missed on the first run, you can choose the episode from the main menu and select "Collector Mode
". Now, you can use a scene select option to browse the locations and choose the area you need.
The following collectible can be found as you enter the dormitories area. First, try to go inside, and you’ll notice you need some keys. Then go interact with the squirrel to chase it off to the totem pole. Then, kick the sprinkler to break it which will make Samuel leave the maintenance closet. Now you can enter the maintenance closet, pick up the keys you’ll need and also draw on the white door!
] - Optional Graffiti #6