Heh!

Nice story!

Almost the exact same thing happened to us at home. We live in a new house where AC powered smoke alarms are required to be put in by the builders.

One night (small-am) the smoke alarm upstairs goes off, full volume. Unsurprisingly we wakt up, and after checking the house, decide there is no fire, but the smoke alarm will not shut up.

So I open it up with a screwdriver, and unplug it from the mains. It stops making loud noises, and goes back to chirping, as per your ones.

Ok, so no AC power, it must be chirping due to the battery, right. Wrong. Even after removing the battery it still chrips every 30 seconds. Must be a capacitor in there? Not that I could see on the circuit board. So here's the alarm chirping with no obvious power source.... A tad frustrating.

I had to take it outside and hammer it to death before it stopped. Weird.

Paul.