Back in the early 70's I was flying for a company near L.A. and every day was IFR, because of the smog... When I landed, I would wipe off the smog from the leading edges of the wings. So. Cal. is a bad place to live...
My white truck shows it all the time. Just clean it and it comes off, but it comes back. Lived all over the US and it's there. CO, NE, IA, MN, IL, MO, NM, TX are all the places I have lived with it and it's been there. Just wipe it off and go on.
About two years ago I had to get the truck re-painted because of Ford's paint job and its inferior resistance to road salt. (I had surface rust bubbling the paint up in at least a half dozen spots) I asked the shop manager about the little rust specks and he told me it was residue from the brakes that lands on the paint and rusts. That's why you usually see the rear of the wheels and across the tailgate, so I was told.
This sounds like a even better possibility due to the fact he has a 10" lift and 38 plenty of opertunity for the break dust to fly up there. Also more break dust created due to the tires and the more weight thrown on the front during hard stops. He works pretty close to some tracks so I will not count that out.
I had this issue on my hood - at close inspection, i discovered tiny peices of metal embedded in the surface of the paint. This is why the rust came back all the time. A clay bar woks nice with all of your weight behind it; however, the only REAL way to get em gone is to turn your truck over to a professional and have them WET-SAND the surface with 1000 grit and then buff the luster back our with compound and a high speed buffer. You can do this yourself BUT use extreme caution and DO NOT attempt to buff around ANY edges.
A couple years ago, i had a car painted in Mexico. i witnessed their rendition of buffing. interestingly enough, it worked great but was pains-taking. They sanded down to 1500 grit and then used cotton stuffing, Brasso, and diesel fuel to buff it back out. it looked to be VERY labor intensive but a safe alternative to a high speed buffer where you could burn through especially at the edges.