When I was a grad student the library offered 'study rooms' to people working on their thesis : you were assigned what was basically a small closet (with a window at least) with a chair, desk, and shelf. The idea was that you could have a quiet place to work and leave your materials there (the room was lockable). Since most students covered up the small door window with paper, you also had complete privacy.
Even though I lived by myself and had ample privacy to masturbate at home, I spent a good portion of that semester rubbing myself to one orgasm after another in that study room. I think it was the stress of the thesis that made me seek relief so badly, but going into my private little room was something I really looked forward to every day.
At the beginning it was subtle hand-in-the-panties type jilling, but after a while I became more comfortable with it and started bringing erotica and (quiet) toys with me. I particularly loved stripping completely naked (even socks!) and riding a suction-cup dildo while I stared out of the sixth story window at the campus below.
Although I tried very hard to be discrete, I'm sure at some point someone must have heard something, and that made it even more exciting.
At the end of the semester I had to give back the key, and I can't help but wonder if anyone else has used that room for the kind of serious self-pleasure I enjoyed there.