I've masturbated constantly for a whole lot of years. Here's a few locations that come to mind immediately, but trust me, there've been many others:
1. Standing at my lab table in the back row of high school chemistry class, hand in my pocket, table edge just the right height for me to frot my cock-head against by way of getting started. Older girl standing next to me had no idea. (Sorry 'bout that, Abigail M.! I'll never forget you, you stupid bitch, I hope you got fucked in the ass good and hard by some guy.)
2. In the chapel of the same school during lunch hour, in a space behind the high altar where I was safe from being seen even if anyone walked in. There was a large painting of Whore Mary hanging there, witness to my orgasms thinking about my female schoolmates. A great memory. Afterwards I could go out and walk around again feeling very relieved, so thank you, jesus xrist, Nazarene bastard, your chapel proved to be good for something besides drilling superstition into us.
3. In my best friend's gorgeous, long-legged, older daughter's bedroom (18+), while inhaling deeply from her used and soiled panties. I'd wanted to fuck this girl for so long and knew I never would. I'd been left alone in the house as a trusted friend to keep it safe. My betrayal of the entire family's trust made me cum so hard I almost fainted! It also relieved the sexual tension I always felt when in her presence, too. I could think: "Act stuck up if you want to, Claire honey. I know just what your cunt and asshole smell like, and I've licked your dirty thongs and cum on your rug." Delightful, just delightful.