Three and a half years ago I installed six hardwired, interconnected smoke alarms, with battery backup in a double house, as the owner wanted. They just called me to say that one was beeping like the battery was bad, so they replace the battery, and it continued to beep. I said it was unusual for one to only last that long but it could be bad. I replaced it with two different smoke alarms( Firex 4618) and it still beeped. I thought maybe I got a bad bath of smoke alarms, so I replaced it with a First Alert, and it still beeps. I have no idea why. I've disconnected all the other ones, thinking another was making that one beep, but no success.

Is it just beeping as if it needs a battery, or is in sounding general alarm?