My early-1980 condo has two electrical boxes for hardwired smoke detectors, one at each end of a hallway and adjacent to a bedroom. I installed one ionization detector and one photoelectric detector. (I understand that newer homes have a detector inside each bedroom.) You're supposed interconnect them together via an additional wire so that if one goes off, they both do. My house didn't have a wire for that.

The round one definitely looks like a smoke detector. If you look at the plastic, does it have 'Do Not Paint' embossed on it?

No idea on the first, but, I agree the round one is definitely a wired smoke detector.

I agree that the round one looks like a hard-wired smoke detector. The square one appears to have lighter gauge wire, which suggests low voltage. It might be good for the OP to take a multimeter to the square one to see if it is getting power and, if so, what voltage. I am also a little puzzled/concerned because there appear to be more wire nuts behind the mounting plate for the round one, but it doesn't look like there is an electrical box to contain them. Lastly, if the 4 screws are removed from the square one, there could be some info printed on the back side of the plate that might help move this investigation along.

It looks to me that the round one is mounted on a rectangular switch box.

Doorbell? We lived in a house for 7 years before we discovered wiring for an old doorbell at a side door.

Nice that they have two doorbells mounted within 24" - both protected by a smoke detector. lol

Thank you all, I am convinced the round one is wired smoke detector. I will un-screw the square plate, see if I could find out something. Is it possible, the spare one is some kind of speaker for the nearby round wired smoke detector, like 30 year old smoke detector setting?

mtvhike- I think you’re right. When I first looked at it, I thought the straight piece on the right was part of the back plate, but it does look like a box. That’s good. :-)

It was probably another doorbell for the side or back door. Can you see if the existing doorbell has the same kind of mounting plate? Maybe they used to be a matching set.

we used to have a hard-wired CO detector that was probably 60 years old. It was like something out of a russian sci-fi movie

Could it be for an indoor siren for a burglar alarm? I have one at my house that is about the same size as the square plate on the left.