Perhaps a friend of yours dropped one of these into your car?
http://www.thinkgeek.com/gadgets/electronic/b278/?srp=3
I have to say, I've bought a couple of these over the last few years. They are awesome. What's even better is that once your "victim" has discovered it, and I've come clean about it, then the victim won't return it -- they deposit it on some other unsuspecting soul...and the cycle begins anew.
You just can't get more entertainment
for months with less than $15.00