Looking at all the plumbing i’d suspect this is your hotwater pipe, i would put a flowsensor here…
@cuprum: @Frans-Jan_Copini has a waterpressure issue, I don’t think it really matters where hot water is tapped, it will always create a problem if you tap hotwater on 2 places at the same time.
I remember my mother opening the hotwatertap in the kichen to get me out of the shower when I was a kid… So that’s proof this issue has been here for ages 
If you are able to get that sensor in your drain… (lift the lid and see if there is space)
The aqara water sensor does allow you to connect wires.
Run the wires to just above the waterlevel.
That waterlevel will rise when you start showering and will lower when you stop.
New idea: I’ll use 2 buttons in de shower cabins. When you use the shower you press them to switch on a night light in both bathrooms for 5 min.
Not completely automated… but should work
Should work as long as you don’t have kids that hit puberty, or get guest that don’t know how it works.
Worst case you could use ‘if lights on in shower 1…’
But would really go for the flowsensor…
You could use a humidity sensor together with the “insight trends reloaded” app. A sudden increase in humidity due to showering reflects in the trend. Same for when showering stops (although with a less steep trend). For sure not real time but should be doable with a 5min delay. Also offcourse dependable on how humid your bathroom gets.
I would go for the flow sensor aswell 
need to gain some trust from the wife first. Not allowed to play with the water pipes at the moment. But in the future… waterflow meter
Now we’re all curious what you did to ‘achieve’ that level of trust 
I have a Hue outdoor motion sensor in my shower and also a meter of my (total) water consumption by smartgateways(.nl) (and additional I used also a waterleak sensor in my shower.)
With these 3 sensors you could use any combination of at least 2 of them to determine if some is in abd/ir using the shower.
Motion + Water consumption total number is increasing OR
Water leak alarm + motion OR
Water consumption total number us increasing +Water leak alarm.
When I’m showering my Wifi speaker volume is increased above its normal volume level (because of the water/shower noise) and my music reacting led strip is turned on (like the music/radio as always I enter the bathroom for more then 2 minutes).
That sounds great! How do you know (fast) that the shower is off?
The water ;consumption sensor has also a value how much liters oer minute is currently consumed so apparently after max a minute I know that the shower is off when it decreases significantly.
Black Iphonitis rigorosa : level 15+
You could save nite the liters per minute value immediately when the motion sensor detects motion in your shower to have the base/minimum value at that time and even if its nit zero (0) you can compare this to the current value so you know when its equal or beliw that value (liters per minute) the shower is off again.
Black Iphonitis rigorosa : level 15+
Or just when the water leak sensor is not sending an alarm anymore. 
Black Iphonitis rigorosa : level 15+
I use a shower head with a bluetooth transmitter powered from the water flow. Shower on bluetooth signal on, shower off bluetooth signal off. ESP32 board running a bluetooth scanner linked to MQTT. Takes about 30seconds to detect that the shower is on. A simple homey flow is triggered when there is an update on the MQTT endpoint, setting a boolean shower_on. There is no off signal, just the lack of MQTT updates, so a simple loop timer that is reset by updates generates that.
Shower heads should have unique bluetooth signatures so it would be applicable in a multi-bathroom setting.
The shower head I use is from hydrao but there are similar devices that should work on the same principal. Espresence to set up the ESP board. Again there are some alternatives.
It sounds complicated but it was surprisingly easy to set up. I’m just using the shower signal to turn on/off some music 
This is very interesting! I’ll look into that aswell
