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There are two cavities in the wall. I want to replace the old smoke detector. Questions:
(1) What's the big metal square thing on the right in the lower cavity, and did the lower cavity once house a more ancient smoke detector?
(2) If I replace the dangling old smoke detector with a new one in the upper cavity, can I patch the lower cavity with drywall? Or does that violate code?
That silver block with two screw terminals is probably a doorbell transformer, which converts (I'm guessing) 120VAC line voltage to the 24VAC that the doorbell needs.
With that in mind, you cannot wall off boxes that contain line voltages; the contents must be accessible. If a box only contains low voltages, then you can wall it off. However, the doorbell transformer input probably must be accessible, and it looks like you have line voltages in the top box as well, which would mean both must be accessible.
