just remembered what I did many years ago - I saw that the car had black woolly seat covers - so I 'got' into the offending car and smeared the drivers seat with old engine oil - you couldn't tell what I had done. A nice little surprise when they got home.
I live in a seaside resort - and 'visitors' from London come and park where they like. I could tell from the open maps etc inside the car that they were not local - by the time they got back to London and found my 'momento' they would have forgotten where they had parked - and I couldn't see them doing a 150 mile journey to try to find out. I do hope the driver didn't notice until he/she got home and sat on their settee.
