Yes it's caused by "fallout" whcih you can get depending on where you park your car, which isn't as scary as it sounds, but it causes little fine rust marks to appear on the paintwork. One way of getting it off is using Meguiars Clay Kit:

and claying and waxing it (you should ALWAYS wax your car after claying it). It will take a good few hours to do, but it will remove the rust marks.