Don't expect anything left in your car to be safe in the future :

Caught on camera: A thief's new high-tech way of breaking into your car - AOL.com

 

Been around since 2013, are two forms but most likely it is a replay attack. Only works with some cars (AFAIK not German mfrs). Detroit recently put together a SIG to study Automotive issues such as CanBus and security

This might be of interest.

 

Thanks - does that mean my car would have to 'friend' your car ?

 

Do these work by copying the signal from your fob when you lock your car? Giving the thief the opportunity to enter your car after you leave or at a later time.
If so using your key would not let the thief clone your signal.

 

Approx 25 years ago in the UK, there was a similar gadget. Although there was no "Keyless Entry" in those days, during the night, your car alarm would sound, you would go out to investigate, turn your alarm off after finding nothing. You would turn the alarm back on and go back in your house. The thieves had a device that recorded and mimicked the remote sounds. Next morning you would find your car gone.
They had used their Mimicked sound to open you car, thereby turning the alarm off, hot wire your car, and drive away.