I should have been more specific... it’s a Smart Things leak sensor... though I now have some from Aqara on order... 
That was an awesome read!!! Thanks for the link! You certainly deserve the title!
Back to the topic at hand... I had forced the re-discovery of one device last night, and this morning it is still connected. Since my Sinopé thermostats are all off right now (turned them off last night. as per your recommendation) I think we may have found the culprit(s).
Not sure if it makes a difference, but I am using this awesome driver from @kris2k2 for The thermostat:
Now I wonder... what next! Can the thermostats be made to behave? Can something be done so the sensors don’t get the booted off when the thermostats are around? Should I order a Zigbee stick? I have never properly debugged a networking issue before... Zigbee or otherwise...
I’m thinking I could turn on the thermostats that is at the end of the house opposite to the still connected sensor to see if it has an impact... would that be useful?
Update: I turned on one of my Sinopé Zigbee thermostats. Now to see what will happen. I had to re-sync 2 of my sensors and they routed through it. The others still seem to go directly to the hub. Will see what happens.
Note: I have found another IKEA Zigbee repeater that I had stashed away. I haven’t paired it to any of my hubs yet... I’m thinking that I will pair it to the second hub unless we can figure out a way for the Sinopé Thermostats not to boot out the sensors... Assuming that this is the issue which, to me, seems likely...
