I had an incident 2years ago when the CO alarm went off at 3 AM . Didn't detect anything but got out . Waited a bit , went back in , opened all Windows and vents , took a look around and could find nothing wrong . Nothing was on except frig . Fortunately was at friends and went to the house till morning . Next day I closed everything back up , and a few hours later took a ride to the local FD . Had them check everything with there meters and found nothing . Checked the CO detector and it was 10 years old . We concluded it had a built in timer for the exparation date .Bought a new detector and haven't had a problem since . It seems that some detectors already have this feature .