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Its carbon/oil reside that is going through the exhaust. even if the vehicle doesnt smoke at all, it can still get past the guides and rings some and give a slight residue on the rear bumper. This is also far more common with vehicles that have exhaust pipes that exit straight out rather than turning down---especially if the end of the pipe doesnt extend to at least even with the end of the rear bumper. The air pressue coming from underneath the car while driving will "swrirl" upwards at the rear of the vehicle causing the deposits to ---well, deposit on the bumper. Its usually far more noticeable on chrome, but if its on your bumper, its on the back of your car.

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That Explains it!