one-too-many smoke detector cavities?

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?