I believe they all beep because they want you to replace all the batteries.

I had this happen a few months ago except it wasn't a low battery alarm. It kept yelling "FIRE FIRE" but then turning off. Of course FirstAlert apparently doesn't indicate which alarm is causing it. They say the initiating one flashes a different pattern but I didn't find one. Ended up, after getting no sleep at all, replacing all 9 of them with nests. At least then if one fails I'll know which one.