There is one or more lines the plumber cannot pressure test unless he is willing to go to the trouble. This line goes upward inside the bathroom wall to the shower. There was a time, before these fancy shower heads, when this pipe never received full line pressure so pressure testing it rarely happened but... some of the newer shower heads have an on/off switch. Pull up the shower switch on the tub spigot then turn the switch on the shower head to off. Line pressure builds in all of it including the drop hose. I know this reads like I visit
www.forplumbersonly. Nope. I knew I had a small leak in the master bath somewhere. I took that wall of the surround down and just could not find a leak. The new shower head turned it into a much larger leak whenever that switch on the shower head was off. I just happened to be in the basement beneath the leak while wife was giving the dog a bath. She was turning that shower head with a drop hose on and off again as needed. Talk about a WTF couple of minutes for me.