Mercedes S-class, heavy duty BMW's, and many others. The system is called Keyless GO
You can keep your key in your pocket and the car will open and lock automatically when you come and go. That's convenient!
These systems make it very easy to steal the car without damaging anything - simply start it and drive away!
What the hackers have is a signal improving device that sends the signal to a second person that has the receiver. This receiver acts like a key.
All the hacker has to do is to stroll along with the car owner so that his device can pick up and send the signal from the key to the partner. It has never been easier!
If you put your key inside the door after you come home; the thief can simply pass your door!
Have a look here (It is in German but you will get the idea)
You can prevent your car from being stolen by wrapping your key in three layers of tin foil so that the signals don't get through.
But that is hardly the state of the art when you have bought an exclusive car....