Smoke Detector ?

I would not mess with it. Toss it out and get a new one.

Only the manufacturer can tell you. Since this is a mains SD with battery backup, that feature may have been designed to cope with power outages: so if there was no power outage there was no occasion for it to send the Dead Battery signal.

The manufacturer can also tell you the probable service life of the unit. This is determined by the radioactive elements it uses, which last for 3 to 10 years. In Canadian units, SDs chirp when the radioactive detectors become weak as well as when batteries are about to expire.

Ditto. It's not working right.

Dave

Hmm. Just a guess:

It needed a new battery (obviously). The AC connections didn't provide power to drive the chirper, but you fixed that.

Further guess: Sometime in the remote past, it began complaining that its food was running low. The then-current resident removed the battery. More plaintive wails from the detector. The resident, thinking "Die, motherfarker" yanked on the ac power enough to quiet the noise.

How old is it? Have you lived there as long as it has been there?

My guess would be that it did chirp and you did not own the house, or you were not there at the time and once the battery got low enough the chirping stopped.

If it is more than five years old, I certainly would replace it. They tend to get old and less sensitive and reliable. I would also replace others that are older than about five years.