I have to agree with the majority here, there almost has to be a removable panel. I had a leaking pipe and thought it was at a exterior cabinet, turns out it was about four feet away next to the WH. The floor of our closet had to be removed to access the plumbing and there was the WH.
In a couple of rigs we have looked at, there were some really ugly looking panels covering the backside of the shower valves. Some are so neatly hidden, you have to be a tech who has worked on them to know where they are.
