My solution is to just change all the batteries at once so i dont have to guess (and if you guess wrong you then you wont sleep tomorrow because of the beeping)
I wish there was some visual indicator to tell which one was faulty.
I bought half a dozen of them to install in our new house years ago, thinking the nightly find-the-beeping-^+=¥€ games were finally a thing of the past.
When one started beeping, I did the rounds, figuring I'd be back in bed in minutes. Well, I wasn't. Turned out the fool-proof indication was to have the normally green LED flash red instead.
Did I mention that I am colour blind?
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.
Manufacturers can get away with it because even if they screw up, an angry customer doesn't really affect their sales in any meaningful way.
A few years ago I used "Find my iPhone" to chase down my phone in the house, buried under some pile or another. Unfortunately I had earlier been using AirPlay to play music through remote speakers, so of course the emergency tone played through the speakers, not the device.