Exhaust Residue

Recently bought '02 GT, with ~3200 miles. It has Bassani X-pipes and AFT-CATs, otherwise stock. Seems to run great, MIL is NOT on, I've noticed after driving a light "dusting" of black dry dust sooty residue on the rear of the car. Looks to me like something that would come from incomplete or improperly burned fuel but I don't know that's the case. Any thoughts would be greatly appreciated.

Thanks but ...

Thanks StangYellow but I'm confused by something in your response. You said what I'm seeing is the result of not having cats but I thought cats were part of the exhaust system that's on this car ... Bassini X-Pipes with AFT CATs. I see something that looks like cats. Did you mean the result of not having stock cats or what?