I second Brian's comments. I had a leak both in plumbing running under / behind the tub area as well as to the supply of the shower valve. Cut all the caulking, remove the yellow insert (if it's still there), drill the rivets, remove the "cover" (this would be where you set soap, shampoo, etc. during showering; on the curb side) and you can then see down to the plumbing that goes forward to the kitchen as well as the plumbing that goes to the shower mixing valve. Replace as necessary. I suggest replacing with PEX and shark-bite (gator-bite) fittings. The -bite fittings can be expensive, but not needing to use a blow torch to install or remove them is a big plus in my mind.