Beeping Smoke Alarm

So, the weekend before last (while we were in Dorking) we popped into Wickes to buy a new smoke alarm, the reason for this being a slightly scary "incident" involving me and some rather burnt supper, and the fact that our existing smoke alarm didn't make single solitary noise during the entire affair...(far from ideal)...

We got it back, put in it's battery, tested it and then hung it on the wall and forgot about it until later on that day when I began to start hearing (almost unconsciously) a little "beep" (just very intermittently)...I put it down to the unit just acclimatising itself, but it was still doing it (with increasing regularity) on Wednesday, and by this stage I must admit it had started to get rather annoying..."busy life (tm)" meant that we constantly forgot to go and buy a new battery for it (as the one that came with it *could* have been duff)..so by Saturday it honestly was lucky it hadn't ended up being thrown into the pond out the back of the house (only the thought that a beeping smoke alarm is better than no smoke alarm saving it's little plastic skin)...

Sunday came and we finally remembered to buy a fresh 9v cell...Flyingpops stood up on a chair, retrieved the alarm and popped old battery out and stepped down to pick up the new one when the beeping sound promptly stopped us both in our tracks...It can't have done that with no power, could it? Anyway, putting it down (partially) to imagination, she put the new one in, hung it back on the wall and we got on with what we were doing (with a shrug)...but meer minutes later the sound happened again...! This, after a moment's pause, sent us running upstairs (feeling foolish) to stand by the other alarm (that must have been it all along!)...we waited...and were finally rewarded, but with a much quieter noise...so it wasn't the alarm upstairs either!! WTF?

The next half an hour was then spent moving slowly around the house, standing very still and listening intently...then moving in, like a submarine captain stalking his prey, I ended up (after a number of false positives) with my head stuck in the understairs cupboard, removing layers of old curtains and car boot sale items and *finally* found the culprit...a very old smoke alarm that had been left by the previous owner that had (*unbelievably*) decided to run low on juice the *very* *day* that we install a new one in the living room...

What on earth are the chances of something like that happening?