I'd try to see if the plumbing in the wall is leaking. See if you can take the showerhead off and plug up that pipe with a cap. Turn on the water and let it sit for awhile. See if you get water leaking out. If you don't, then your plumbing is probably good. Look for cracked grout in the tile, or areas that the caulking has degenerated and fix that. If neither of those help, it is time to start tearing things apart. Hopefully, you have an inside wall where you can cut out the drywall without messing up tile, etc. and try to see where things are leaking. Good luck...My unprofessional opinion.