I used to sell hottubs for work and this one day it was particularly slow going. I was sitting in the lounge of a tub, reading, and the two guys I worked with were behind the counter on the computer.
I thought it would be pretty hot to touch myself with them so close, so I slipped my hand down my pants and started rubbing my clit. Knowing that at any moment either of them could walk over and see me got me so hot.
I could hear them talking-only about 10 feet away-and I could feel my orgasm building. I went a little quicker and I came silently inside that hottub.
I don't know if either of them noticed, but they never said anything. To this day it's one of my most thrilling masturbation experiences.