I don't know if women experience this, but as a guy I've noticed this thing I call "The Shitter Sniff". It works like this. Invariably, every time I walk into a bathroom and someone is birthing a litter of tapered brown snakes in a stall, they will, upon hearing the restroom door open, elicit a loud 'sniff' as if to mark their territory. It's audio shorthand for "Don't jiggle the stall door man, I'm busy in here."
Guys at the urinal do no such thing as there is generally very little worry about occupancy confusion.
So two things. Guys, pay attention the next time you waltz into the bathroom. I guarantee if someone is copping a squat, they will either sniff, cough, or make some sort of noise to alert you of their presence. And Ladies, let me know if you notice the same thing amongst your peers.