Found in the Incheon (Seoul, South Korea) airport . . . I pressed it and nothing. Tried in several bathrooms - I had a 10 hour layover, so lots of time to test the facilities! Never got any result.
Googled this - it's supposed to make a loud flushing noise to cover up any embarassing bathroom noise.
Did I mention - Incheon is the only airport where I've been under-dressed - I apparently left my skirt, blouse, pantyhose and heels at home!!!