I put smoke detectors in my previous house (all battery operated) but by code, my new house had six of them hard-wired in.
Silly me, I figured since they were hard-wired, they didn't NEED batteries . . . and since the manuals weren't included in the packet of stuff the builder gave me, I never gave it another thought.
Eighteen months after I moved in, without any preliminary chirping - all six alarms sounded.
At 2:30 AM.
At first, half asleep, I thought it might be the burglar alarm . . . but I'm happy to say my first thought was "DON'T SHOOT A FAMILY MEMBER!!!"
Then the alarms stopped . . . only to start again after 10 minutes. So there I am, running around the house looking for a fire, trying to smell smoke, even checking the attached garage and going up to the attic to make sure nothing was smoldering up there.
After a couple of on-and-off cycles, I used a stepladder to reach the detectors . . . and found that they indeed, did have batteries.
I change the batteries every year now around Mother's Day, even though a voltmeter still indicates they're OK. And I replaced all of them a couple of years ago - they only last about 10 years, and when one goes bad, all six alarms go off at (you guessed it) 2:30 AM.