We have a series of Kidde 1235 smoke alarms hardwired in our home. Last night 2 of the alarms went off and wouldn't be silenced even when the doors were opened. The alarms were in two different rooms that utilize two different air conditioning systems. There is no source of combustion or contaminated air that we can find. The detectors are approximately 12 years old. What can set these things off ? Do these units have a service life ? Can they be tested ? Would appreciate your insites. Thanks, Gary