Empty is not the same as clean
Two things happen in a house that has been closed for a season, and only one of them is ours to put right. The first is the dust. In a lived-in room the air keeps moving and the film stays loose. In a shut one nothing disturbs it for months, so it beds down on sills, blind slats, shelf edges, and the tops of doors into a layer that has to be lifted with a dry pass before anything damp goes near it. Wipe it wet first and you have made a smear you then have to remove twice. The second is the plumbing, and it is not a cleaning problem at all. Every sink, tub, shower, and floor drain holds a small pool of water that blocks sewer gas from coming back up the pipe. One plumbing manufacturer puts that evaporation at roughly two to four weeks of disuse, after which the seal is gone and the drain is an open channel. That is the sour smell people mistake for a dirty bathroom. Running each fixture for ten seconds refills it, which we do as a matter of course. If the smell survives that, it is a plumber's question and we will say so rather than keep scrubbing.