We had this on the Tradsong e-mail forum - it was judged to be an urban legend.

My opinion is if they want to steal a car this is believable. There are other methods like rocking the car while the mark is reversing close to other cars, and dropping a bag of cans in sync. The essence is "distract the driver". I have seen the latter as a Candid Camera type wheeze on TV.

If you have a car worth stealing - remove the keys when getting out. Petrol pumps ditto, lock the doors when you go to pay.

My Bodhrans are arguably more valuable that the car and I don't want anyone getting in and leaving a couple more in there.