Learned my lesson after a trip last week... I have sensors for nearly everything, but somehow totally forgot about the Fridge / Freezer.
A power outage made my fridge lose it's mind and turn off cooling, even after it powered back up. Unplug / replug seems to have fixed it, but all the food was spoiled when we got home. Simple $10 temperature sensor could have saved everything!
I definitely have the water sensors covered, even have a water shut-off valve tied to them (that's the one I'm most paranoid about).
Interesting idea w/ the smoke alarm 'relay'. I have some echo dots, that in theory are supposed to pick up smoke alarm sounds when in 'guard' mode. The water sensors I have throughout the house also have temperature sensors (and I have others from ecobee), I have those send an alert if they ever get way too hot or too cold, and announce that over speakers. In theory that could also help with detection of fires or failed HVAC, etc..