No previous leaks were found, but one morning the kitchen floor (on the other side of the bathroom wall) flooded when the shower was turned on. It's a high-usage shower, used for a minimum of 2-3 showers a day. Suddenly it just started flooding, but the bath is not showing any signs of a leak.
What is the most likely issue? Could it be the pipe above the faucet?
The easiest and first thing to check is the trim pieces around your valve knob(s). These trim pieces usually come with a cheap foam gasket, which crumbles apart after a while. Then water runs down the wall, gets behind the trim, and goes into the wall.
When you take the trim pieces off, if the gaskets look bad, scrape them off and replace them with a bead of plumber's putty, which works much better than foam gaskets and lasts forever. Form the bead all the way around the trim, but leave a little opening at the bottom of the trim; that way, if some water does get behind the trim somehow, it will have a chance to escape, rather than accumulate behind the trim.
Remove all the trim covering the valve assembly/s (handles, escutcheons, trim plates, etc). Inspect (with flashlight) for signs of leak source and moisture. Turn valve/s on whilst inspecting to see if there is a leak associated with the actual valve/s assembly/s. Look for source of water. If you cannot at least get a general idea of where the leak is coming from I am afraid that you will need to open the wall on the opposite side (kitchen?) to create an access point from which to conduct a more thorough inspection. Cut an inspection hole and take a look. If you then provide pictures of the leak mechanism then many helpful persons on this site will guide you through the repair process.
