Tom, Steve is right. Like with a leaky roof, the results of a leak can show up far away from the actual leak. Water can travel along the lumber in your walls so you really have to start with removing the wall covering. If you have a panel that you can remove to check the back of the valves, check that completely first. If not, make one. The next thing would be to remove the ceiling below. One thing that is weird enough to happen and will drive you crazy is condensation from a bathroom vent fan. It could be puddling somewhere and the weight of someone standing in the shower could allow the water to start moving.