I have a mains smoke alarm with a 9v battery backup. It started to beep:
regular beeps at roughly ten second intervals for something like a minute,
then a long silence before it started again.

According to the manual this is the battery-low warning, so I took off the
cover, removed the battery - and then discovered that I didn't have a
replacement.

I replaced the body of the unit without the battery - and it's still
beeping. Not regularly this time but seemingly at random: beep... beep
beep beep... beep beep. Again this is for about a minute at a time.
There's nothing in the manual about this and it seems too irregular for a
built-in "no battery present" warning (and if there was one, I imagine the
instructions would mention it).

I don't want to turn the alarm off overnight but I think I might have to.

Has anyone else encountered this or something similar?

Thanks.