I used to sell new cars for a living a few years ago and we used to have problems with this, we used to call it factory fallout. My understanding of it was that it was tiny metal particles sitting on the surface of the car and often staining the car with rust rather than the cars paintwork rusting through. New cars go through such a process in the build that i would be surpirsed if it was rusting from underneath. As far as i remember, the valeters used to clay the car and glaze it and sometimes machine mop it if it was really bad.
As far as your dealer is concerned, if that is a new car, it will have an anti corrosion warranty (not sure what vw's are these days, but i would put the responsibility (and cost) on their doorstep to put right.
Rant over, good luck.