In the bulletin post at twitter post of SPPD, they included several photos of the car itself and if you look at the rear bumper closely, you can see that there is an area of the bumper where it seems clean... You can see where the dirty part ends and the "clean area" starts, which in my mind, is very consistent with something big being pulled out of the trunk...
Everything regarding cleaning and disposing of evidences seems very hastily done and in a major hurry... which definitely would work towards achieving justice and closure for Kira and her family...
Maybe when the trunk liner was taken out...it brushed some of the dirt off of the back bumper? If he wrapped Kira's body in something i.e blanket.. it could have also did some brushing off of the dirt.