On one outing last year I left my kayak on the roof of my car for a 6 hours or so. I was out of viewing distance and the yak was ripe for the taking. Keep in mind that my kayak is sort of beat up and junky looking. I do not ever expect anyone to try and steal it.
Anyway, by biggest concern has always been that some punk kids would try to steal the boat and damage my car in the process. I bought the yak for $75 (a steal for this used boat) and I would not mind all that much if somebody stole it as long as my car was unscathed in the process. I have contemplated writing the following words on the bottom of the yak (which is clearly exposed when you look at the yak on the roof of my car):
"If you plan to steal this kayak please take care in making sure that you do not damage my car. Thank You."
If you ever see a kayak with those words scrawled across it - odds are that it is mine.