2006 Sonata Observation, any corroboration?

Been noticing an accumulation of dirt over and on the bumper area in the rear. Was driving behind one the other day in the cold and I noticed that the exhaust vapor kind of gets trapped in a dead air space behing the car at lower speeds, curls up and seems to get trapped up in that area. Now certianly this may just be drainage and general road grime due to the onset of winter, but it does look like on the top bumper surface that a fine layer of dirt/grime is being deposted by some process other than rain or road spritz, anybody else notice this?.....Doc