Carbon monoxide detector issues

[MiniMe]

Not sure if this should be posted under Electrical or Appliances, but has anyone had issues with the stock Safe-T-Alert CO detector? The original one that came with the TM started alarming a while back on a trip during the night while using the propane heater. We/dog woke up, felt fine, didn't smell/notice anything. Aired the TM out and shut off the heater/tanks for safety. I had the dealer replace the detector with a brand new one when we had a servicing/maintenance done. I put the unit in our detached garage and I could later hear the detector intermittently beeping, sometimes alarming inside. Our TM is always stored with the propane tanks shut, toilet/tanks empty, interior cleaned and there are no gas appliances in this garage. I had to just disconnect the battery completely to stop the beeping as it was loud enough to hear outside the garage with the door shut. Then I disconnected just the detector wiring from battery power (so I could still charge the battery) and purchased a plug-in wall mount detector for when we're actually using the TM. Neither the dealer nor Safe-T-Alert support could help me troubleshoot so that was about $75 down the drain. It's been a while so the "new" detector is out of warranty, but if anyone has any ideas of what else could possibly have caused this issue, please let me know as none of the typical causes apply to my situation.