I've been through a couple of mining towns in the American West, and seen cases where the whole town consists of mobile homes on cinder blocks, ready to pick up and move on short notice.
I also happen to live within reasonable driving distance of Centralia, PA, where the coal mines beneath the town caught fire and the town was abandoned. They've razed most of it over the last couple of decades, but it used to look pretty creepy with all of the boarded up or broken windows, overgrown lawns (or else scorched), and warning signs posted. At one time you could occasionally see tongues of flame leaping up through cracks in the road or pavements, but the fire has long since burned out under the town and migrated a few miles away, although the ground is now unstable.
I would assume that the abandoned towns/cities would be looted over time (and some buildings cannibalized for materials), so there would be very little left that wasn't either permanently secured or simply not worth carrying. Most archaeological sites of ancient cities find very little from the end of the city, since those would typically be looted, but a lot of earlier materials which were lost and buried under later developments or thrown away and buried in garbage piles.
