There are occasional places that are all destroyed - easy to miss in fields, but obvious if it’s on a road or in the forest. In a field, there will be no grass in a square area (say, 15x15 or so?), in a forest, everything in that same sized square will be destroyed, so lots of heavy sticks and splintered wood where the trees used to be. On a road, it will all be rocks and rebar, same size.
Anyway, those things are radioactive some portion of the time (half-ish?). Most obvious thing I can think of.
Otherwise, best guess would be an artifact (the area where a “natural” artifact spawns is occasionally radioactive, too) or a damaged atomic something (nightlight, lamp, coffee maker, mini-nuke - they change to a green color when damaged to show they are leaking radiation).