Once, a co-worker swore this happened: End of the night, the movie theater pretty much closed 10 minutes ago. An attractive, tall girl about 20 y/o had sat, and put her back up to a doorway in the middle of the hallway, masturbating, with some success I'm told.
After being caught, her expression slowly went from "I'm going to come any minute now", to "Oh, shit I've been caught!" She fell, getting up and trying to pull her skinny jeans on at the same time. She fell a second time, then decided the ground was as good as place as any to hike drawers, and run like hell. The custodian had turned his back after glimpsing her, and had heard low insane mumbling, and two hard whacks to the wool carpet. At least she said "Sorry" before damn near knocking him down on her way out.
Sex by yourself in a public place is definitely a sad thing, but for bored lovers or desperate sluts, it's fair game, so long as they don't get caught... or leave a mess. Or break something. Or scar someone's eyes for life.