are you sure? How does the car know the fob is in your pocket without sending something?
I was told it's because the car computer is constantly pinging for the fob. Once it realizes it's not in the vehicle it does it's magic. If no signal is sent, no signal can be stolen.
A much more scientific hacking method can be used to get into your car computer and do all sorts of weird stuff. Saw a news story concerning police departments trying out computer systems to search for specific cars. Such as one that's been stolen. Has the ability to ping that car and disable it.
Pretty cool stuff.