The battery ran out on my sensor and it wouldn't be found. I tried resetting the sensor (removing battery, holding reset button for 6 seconds, etc...). The sensor is now flashing red/green. I've tried re-adding it with no luck. When I've tried to remove it from Hubitat, after hitting remove nothing happens.
I've tried resetting the sensor numerous times and tried readding it skipping repeaters etc...
Shutdown the hub and remove the power for 15mins. This should cause all your zigbee devices to go into panic mode. Power up the hub and try again with in 3ft / 1m of your hub.
You might try physically moving the Zigbee device in the same room as the hub to re-pair if you haven't already. Once it is re-paired you can move it back.
I have had several of my beloved SmartThings Motion Sensor V3 devices just stop working despite battery changes, resets, etc. I keep them in a box in hopes that they come back to life, though.
It may also be helpful to state the specific model number of these, as I think there are at least five or six, and people don't always agree on what to call them...