Yes, it is paint overspray, but not just from painting your car. Little drops of paint that get on your already smooth paint would be overspray. Maybe you are confused because people mention getting overspray on their cars while it's parked or something (not at a paint shop). This can happen from any sort of paint. If some building is having repainting done, oftentimes overspray will get blown all over the place. So you could be parked at work, the building next door is having something painted, and you could get overspray all over your car.
I suppose the term could be used when spraying anything, though. I guess if you were spraying your wheels down with wheel cleaner, and the wind blew it all over your car, you could probably call it "overspray".
