Pull the shower handle assembly
While replacing my shower head with a Bricor today, I noticed the entire shower handle assembly was loose (shower has never been used). Both PEX connections to the assembly were loose, but were not leaking.
When I unscrewed the assembly to tighten it from behind, I noticed that the blue (cold water) PEX tubing was stapled via a paneling staple. Sure enough, it was spraying all over the inside of the wall AFTER the PEX tube was disturbed from me adjusting the mount to tighten. It probably had a much smaller / constant leak prior.
In any case, 3 years later and still battling quality control issues. Never a Thor again...