Posting this as I am sure I have seen others ask, then I had the issue and finally cracked it!
My issue was I was seeing a random device, a light in my case, being switched on, for no obvious reason. I woudl come down and find the WC smart light bulb on. It had not been switched on in an expected way so all automations ignored it. It would stay on forever.
I solved it temporarily by adding a BUG FIX automation. One to solve it until I figured out why it happened. That BUG FIX was just to monitor it for turning on and then run a timer before it turned back off. Which would be longer that the light would likely be on through other means (motion/occupancy etc).
FULL SOLUTION!
Finally figured it out, by chnace really. I nticed it had turned on again, so i looked to see what else had happened. We have Alexa devices and each room that has an Echo is setup in that Amazon App so that the rooms light is in the room of the Echo. So Bedroom has Bedroom echo, Bedroom Light. Bedroom Radiator TRV.
The WC is the last light t have been added and was the one that would randomly come on. The reason was it was somehow being picked up by Alexa as linked to Bedroom. So in that room, “Alexa Light On” would correctly turn the bedroom light on, but also incorrectly would trigger the WC light too.
No idea why. I think that might also be the last Echo to have been added (or replaced). So probably last Echo and last light in house were being incorrectly linked without it showing up.
Solution was to put the WC Light into its own room in the Alexa App, even though there is nothing else to join it and nothing else to control it.
Looking through, other devices or lights that are in the Alexa App and not in a rom seem unaffected. Solving the WC Light hasn’t shifted the issue onto another device. But it has solved the issue.
Posting it as it might be useful to someone else in future. I am sure I have read about this weird issue before.
Cheers
Rich