MYSTERIES OF TECHNOLOGY

There's a device somewhere in the bathroom at my office that makes a sound exactly like someone breaking the seal on a day-old 3L bottle of soda and twisting the cap twice. I don't know what it is, because by the time I think to look for the source of the sound, it's already over.

(TSS! Rack rack.)

One possibility I've already crossed off my list: Someone in another stall breaking the seal on a day-old 3L bottle of soda and twisting the cap twice. The sound can happen when nobody else is in there but me.

Any ideas? This isn't an emergency or anything, I'm just curious.