one of our gas engineers got called to a job on monday, CO alarm going off, Transco had shut boiler off, he visited, fully checked boiler all ok so changed alarm, got a call on tuesday saying new alarm going off but only once per minute, guy calls back and sure enough he can hear a peep every minute, assuming new alarm is faulty he changes it, but can still hear a peep so he takes both alarms outside into fresh air, no peep from either so thinks mmmmhhh could be low level CO or something, so he goes back in with FGA to check boiler again, and can STILL hear peep, turns out tenant had put smoke alarm in kitchen drawer because it was making a peep but didn't think to tell my guy about it, DOH