FYI all, I purchased one of the Sinope water flow sensors, cut the end that plugs into their water valve off, and there are 3 wires - black, yellow, and red. With yellow in S1 and black in C, it is indeed registering "present/not present" when setup as a motion sensor with water flowing/not flowing. So, for the purposes of knowing if a shower is running or something similar, it looks like this would indeed work if installed on the pipe between the thermostatic valve and shower head/hand shower, etc. I will say though, when testing it with just a small flow of water under the sink, it didn't switch from not present to present... but, with a steady flow like you'd have in a shower pipe, it switched immediately. So, if you're hoping to monitor for a small amount of flow that could indicate a leak, I'm not sure this would work. But, if you're just hoping to monitor whether a specific pipe has flowing water in it or not, then this application could indeed work!
My plan is to do what I stated before - install these on the 3 bathroom showers' plumbing in my home when I redo them to know whether someone is indeed showering or not so I can include that in automations (e.g., don't turn off lights since motion isn't detected, turn on the shower light specifically, maybe turn on a Hue scene for the light?). Even better, I can also install a water leak sensor with the same ZEN17 and monitor for potential leaks down the line too. 2 birds, 1 ZEN17 universal relay 
