This practical joke is best done to people who are truly worried about their car, truck, whatever. The type that parks their car 20 hectares away from the mall to avoid it getting touched by human kind.
When your buddy leaves the car, get your friend to place a note on the windshield that reads "Sorry, about the scratch. We will take car of any damage." and add a fake phone number. Then as you come out with your buddy from the mall, and they read the note, let the search for the non-existent scratch begin.
You can help by point out "potential" scratches, watching him or her run over to see, only to realize it's just dirt.
This one can done from afar so that you can have practical fun watching the owner of the survey for damage rather than you helping out, can be even more hilarious.