From the internet: If you have a wireless key fob for a car with a remote keyless system, then you might want to start keeping your keys in a freezer or other Faraday Cage to protect it from high-tech thieves, who can use a $17 power amplifier to break into your vehicle. Cars with keyless entry systems are capable of searching for a wireless key fob that is within a couple feet of the vehicle, but car thieves can use the “power amplifier” to boost the key searching capabilities, sometimes up to around 100 meters, and pull off a high-tech car break-in.
So if your key-fob is sitting on a table within 100 meters of the car, you are at risk.
this apparently only works for keyless entry systems, where the car searches for the fob. Older systems like the one I have, the unlock sequence is initiated by the key fob, not the car.
The best way to defeat this, is to “put your keys in the freezer, which acts as a Faraday Cage, and won’t allow a signal to get in or out.”