This is earned for finding optional collectible #3. If missed on the first run, you can "Play Episodes" and select specific parts of the Episode in order to find the missing optional collectibles through collectibles mode
. The following collectible is described and can be seen in the video below at 3:39:
Upon reaching the gas station, go inside, and look on the window ledge nearest to door to spot some stickers. Pick one up to unlock this.
Getting all 6 collectibles in this episode will also reward you with the following: