Harry Bloomfield explained :
6am this morning, the beep-beep-beep starts again. Just a short series, still
not long enough to be certain where it is coming from, but....This time I
think I know where it is actually coming from. A battery powered fire alarm
in my loft workshop/radio shack.

Nope, still wrong. The one in the loft doesn't have a replaceable
battery at all. It says it is good to 2024. So now I am completely
stumped as to what is giving out three beeps at very irregular times.
If only it would repeat its beep long enough to track it down.