You'll never trace this one 
I just rebooted my system, as I do when playing with WebCore code.
My bathroom light didn't comes on so I assumed ZW was just lazy after a reboot.
Walked in again after a minute and no light/fan again.
Put the trace on the piston and no event, Hmmm.
Looked at the device, SmartThings motion sensor, only to find it was WET! WFT?
The driver had changed from Generic ZB Motion (no temp) to Generic ZB Moisture (no temp).
I wasn't changing any drivers just WebCore code.
It was working fine until I rebooted.
Ghost in the machine.
