One of them starts complaining about a flat battery, so after tracking down where the noise is coming from I trip the breaker for the smoke alarms. At which point it stops complaining, but continues to flash the "I'm working" red led every so often.
Flick the breaker back on, and the green power led lights up... and it doesn't start complaining about the battery.
Until about 10 minutes later.
Edit: and now after replacing that battery, at least one other one has also decided that its battery is flat. And of course it shut up as soon as I got up to try and track down which one it was.