Any car can get overspray, and it can come from a lot of places. Certainly it can happen in a bodyshop, and not just from the spray booth, but they spray other stuff there during the course of repair and not just in the spray booth.
Mike Phillips likes to say that "car guys" get overspray on their own cars because they are always painting something in their garage. As David Hayward mentioned it could be from your neighbor doing some rattle can or HVLP work, but it could be from a pro painter doing something on a house/fence, to driving under a bridge that's being painted, or any number of industrial activities from painting the outside of buildings to roof coating, etc.
That's the classic really, where they are doing some painting/roofing on an office building and the overspray blows over to hundreds of cars at the next building.