Do we know where this drip is coming from other than behind the shower?
FWIW- I had a slow drip from the same area. I removed the chrome trim plate from the shower valve and found a male threaded adapter to PEX in there the was defective from day-one. The adapter never finished the molding process and there was a big chunk of plastic missing along the threads. Those morons on the assembly line, rather than walking back over to get another one, simply hooped pipe dope all over it and threaded it in.. and they wonder why we complain about workmanship.
In the end I went to HD and bought a small non-ratcheting PVC cutter, sawed the handles off to about 2" and was able to get in and cut the PEX low enough to splice an extension and get it back to the NEW, FULLY FORMED adapter. 'Twas a beautiful thing but you better have skinny arms and spare blood because both are required