Ok so I looked through all of my
Amazon orders and it was 2016 that we bought the chirpers. TVGuy, I considered just skipping the whole idea of hard-wired and go back to the old fashioned battery-only type but that would not pass code if we ever decide to sell our home. Fire inspection must be done before settlement so we either fix it correctly now, or do it later. Who knows when we'll ever move but chances are good that an electrician would cost much more in 20 yrs. At least that was our reasoning.
We decided to try one last effort before calling an electrician. When dh held down the reset button of the chirping device, he did it that one only, not all 4 of them. So we decided to pull all 4 brand new ones and reset all 4 again, even though really, how could they need to be reset already? Only a few hours out of the box? STILL THE FOYER WAS CHIRPING!!! ARRRGH! omg I had to leave and go for a drive before we kill each other.

We were going completely mad!
Well, it was all resolved when I got home and you wouldn't believe what it was.
The stupid CO detector plugged into the outlet in the foyer which also has a back up battery .

(Yes, I know in reality, they're not stupid and I'm glad humanity has this technology.) It's not a huge foyer but the way it echoed sounded like it was coming from the smoke detector. And I guess what we thought was a different one chirping after the first one was pulled out, must have been the CO detector all along but I could swear the chirp was coming from down the hall where bedrooms are.
Whew! Poor dh, he had all 8 of them in a box, put it in the garage and still could hear chirping in the foyer. Kind of hilarious now, in hindsight.
Thanks everyone! Check ALL of your detectors! and have a great weekend!
BTW, the reason they only last for 10 yrs is that they use a TINY bit of radioactive material as part of the detection, actually less radioactivity than what the earth itself naturally emits so don't get all crazy over it and toss your smoke detectors. We're all exposed to the earth's radiation so nothing we can do about that.
ETA: Ah Deb, I see you beat me to it.