I was awoken by the "chirp" in the early hours of this morning. We have 2 mains- powered smoke alarms and the chirp seemed to be coming from the downstairs one, so I disconnected it from the mains and set it on the coffee table and went back to bed

chirp

aaaarrgghhh

Back downstairs to remove the backup battery from the bloody thing. Then back to bed

chirp

aaarrgghhhhh

I stood under the upstairs one for 10 minutes daring it to chirp. It didn't, but the downstairs one did. Despite having no power or battery in it.

Odd. I can only assume that either it had enough power in a capacitor to chirp, or I have a poltergeist.

It's now silent, but I missed out on a couple of hours sleep, so am grouchy today.