I’m not thrilled about “staffed/unstaffed” because that implies that there should be more than one person sitting there and all of them were out. Staff represents a group of people.

I’d see staff being used in the case where somebody walked in to an office with several desks, and there was nobody around. Then saying the place was unstaffed would work.

Perhaps not politically correct, but I do not feel diminished in any way by the words manned or unmanned. It’s one of those words that, to me, does not specifically imply that the person was male, like the word mailman does.