I had an interesting theory mid year in 2022. The though being that you should be able to theoretically find out when you shower based on measuring the temperature of the water mains line (talk about shower thought).
The theory is simple, if the incoming water temperature is not at the ambient temperature (ambient being not real ambient but rather the temperature between the wall and concrete of the house), then you can tell if there is a change in flow rate by a change in temperature. This is based on the fact that if there is no flow, the water pipe temperature will reach equilibrium with the environmental temperature and any flow from this point will lead to a change in temperature.
I strapped a temperature probe to the outside of my water mains over the last 6 months to better understand this and have found very compelling results. In fact the latest trails today proved out my exact theory.
In the chart below you can see a few different stages over the morning. The first is when my heating comes on in the morning to heat up the house when I wake up (temperature set point is set to 21.5C in the morning so I wake up to a warmer house). I then started to shower at about 6:50am which resulted in an almost immediate drop in incoming water temperature leading credence to my theory. Once I finished my shower the water temperature began to stabilize again and I notice another suspected shower event where the water temperature shot down again once again leading to my theory of being able to identify showers based on water temperature.
The big learning lesson from this is that you may not always be able to measure your desired measurable but in any process there are always a lot of intertwined variables. Here we may not be able to measure direct water usage but we can measure changes in water temperature with the assumption that these are coming from large volumetric flow rates.