Okay, I have to admit I did something similar this past Christmas. My DH and I were in Macon, GA for a shopping weekend and stayed overnight in a hotel. Well, the next morning we were leaving and parked right next to us was an SUV that had DVC stickers on it and a firefighter Mickey on the back window and a DVC thing around the back tag. I think they were from SC.
Anyway, I had happened to buy the bag of four Christmas toppers the weekend before in Disney. I noticed they didn't have one on their car, so I grabbed one and my DH put it on and I wrote a note saying Merry Christmas from one Disney fan to another and signed it dhcoffey on the DIS.
Never heard anything, so I don't know if they DIS or not but it was fun. I even wrote my note on lime green sticky paper I had in the car.