Years ago a friend of mine was showing me his Mercedes or BMW that would turn on the parking lights and dome light and unlock the door when you got it's fob about 6 feet from the car. It wasn't a key fob that needed buttons pressed. The fob didn't need to be waved in front of a panel. The great thing about it is that it would lock once he stepped away.
Any idea what sensor it was using to do this? Is there something available for the Arduino?