Is This Soot or What?

Take a look at the attached photo. It is a close-up of part of the hatchback. I noticed some black stuff it when I was getting things out of the car. Earlier in the day I got a free "hand" carwash from the dealer as a birthday present. My disappointment with the carwash is another story.

Anyway, in the afternoon I waxed the car with some good stuff, not the bottle of sealant cleaner that came in the glove box. I used Zymol. So, my care was clean and waxed Blizzard White. When I took the stuff out of the back of the car in the early evening, I noticed this black stuff across the back of a strip of metal one the hatchback. I ran my finger across it and it appered to be a substance like soot. Where did it come from? It was a bright, dry day so it did not come from a pothole full of water or water on the road. My guess is that it may have come from the tailpipe except that the pipe is on one side of the rear and the stuff was evenly distributred across the back. The other guess is that there was a static charge that attracted the material.

Anybody have any other ideas?