I work as a secretary in a university department in the West of England. The ladies toilets on our corridor are unusual in that there are two cubicles, back to back to each other, so you have to walk around past the wash basins to get to the far one.
On one occasion I was in the far cubicle when I heard the outer door open and then someone entering the cubicle nearest the entrance. I have to confess I am mildly aroused by listening to other women peeing, so I listened as the other woman sat down, keeping very quiet myself. She had a pee, then there was a pause, and I expected her to get up, flush and leave, but all remained silent. Then I became aware of little noises, and I realised that whoever it was, was masturbating. At first I could just make out little rubbing noises, then slight gasps and heavy breathing which was getting faster and faster. After a very short time, maybe only a minute or so there were a few seconds of total silence followed by a couple of stifled, but still very obvious gasps and I guessed that whoever it was had cum. I was torn between the need to cum myself, and the desire to find out who it was in the next cubicle, so I very quietly opened my door, made my way round to the wash basins, and started to do my hair in the mirror.
A few seconds later I heard the toilet flushing, and the door opened. I looked in the mirror and was shocked to see that coming around was one of the senior lecturers, a woman of round my age, with a humourless, dried-up reputation, not someone you'd think would be overtaken with the need to masturbate in the middle of the morning. I briefly nodded at her, finished doing my hair and went off back to the office. For the rest of the morning all I could think of was the totally unlikely image of this apparently sexless woman sitting on the toilet and frantically rubbing herself off. Within the hour I was in the toilets once more, bringing myself to a very quick and much-needed orgasm.
I'd be very interested to hear of others who may have overheard women masturbating in public toilets, I'm sure it goes on more than we think!