Our upstairs bathtub has a small leak inside the wall that over time softened the ceiling sheetrock below. After removing the sheetrock we tested the shower, and it's weird -- it leaks about 1/4 cup ONLY when someone is IN the shower, and ONLY when the shower is turned off. If we run the shower with no one in it, it doesn't leak no matter what. While the water is running, it's fine. Only leaks with someone in it and only at the moment it's turned off.
I've removed and re-caulked the tub spout/diverter and the valve decorative flange.
It's a 20 year old Moen single valve.