The thief sign is a junk item. It's only purpose is to shame the player for the crimes they've committed.
Whenever the player manages to take an item from a Petrol Station or Outershop without paying for it, and then saves their game to go back home, they will receive the thief sign. Everything on the Roof Rack and Trunk will be replaced with the thief sign and two Trash Bags in the trunk, as penalty for the theft. Components installed in the car will not be affected.
It costs nothing, and cannot be found anywhere.