docshell:
This is why people should have hard wired alarms not battery operated then they won't fail and even go off if the elec cuts out just in case!
When I moved out of home, I insisted the council change my smoke alarms to hard wired ones. Trying to get my Mum to get them, as hers keep beeping, and so she takes the battery out. As she's a smoker, who also smokes in bed, I can't bare the thought of anything happening. Especially as she call fall asleep on a clothes line. 
So glad they got out. The alternative is a horrendous thought.