I suspect that they are both smoke alarms, but the newer one might also be a CO monitor. Demount each one so you can read the label -- a simple twist -- and, if the label doesn't provide all the info you need, Google the label info to find out, or call tech support for the manufacturer(s). Do they both work?

It looks like a smoke alarm that is more than ten years old. Take it to a fire station for verification and advice for disposal.

You may live in an apartment or condo. If so, the old one has a strobe, so it is probably wired to the whole building. The newer one is probably battery operated and as said probably a combo smoke/CO for a bedroom/hall.

Yes it's an old smoke alarm. It was originally white but the plastic yellowed over time. It is just battery operated, not hard wired plus battery operated like the newer one. I would guess the previous owner didn't want to have to fix the wall behind it if it was removed so just left it in place.

PRN-

That old one is likely held on with 2 screws in plastic anchors. Easy to remove and spackle if you don't need it.

Kudzu9,

I agree, but you know how these things go. The paint on the wall behind the smoke alarm is probably slightly different by now due to fading. Not everyone has the ambition or even rudimentary skiils to fix things which could explain why it was left as is.

@PRN are you the OP under a different username? Otherwise, how do you know which one is/isn't wired?

It looks like the beginning of a smoke alarm museum.