This was covered in depth on the Security Now podcast in May 2015 (Transcript [1]). The Passive Keyless Entry and Start PKES system relies on the assumption that if the car can "hear" the key, the key is in close proximity. Normally that's true but it is technically trivial to build a radio system that picks up and amplifies the car's continuous "ping" transmissions. So the key, which might be in your pocket in a restaurant, hears its car's ping, it responds, and the bad guys pick that up and amplify it and the car says, ok, key is here, open the door.