Maybe it was a case of mistaken identity. The person placing this object/symbol thought he/she was placing it on a car of a friend/associate (same make, model, color, etc...).
In the late 90's, I had someone place a phony parking ticket on my car (parked in a college student parking lot). Clearly it was meant as a prank for someone else who also was driving a late 80's gold color Mazda RX-7 because in it was a note to its intended target. Still I took it to the campus police as my sister worked there at the time.