There are these microwave sensors in car alarms that have dual zone. This means that they can detect movement both outside (a small distance) and inside of the car.
How are they implemented?
I suspect two regular microwave sensors that have different powers: one with high power for outside detection and one with low power for inside detection only.
Edit: link to a dual zone microwave sensor: http://www.crutchfield.com/S-D98ka8tAZol/p_220MV3/Code-Alarm-MV3.html. It's a single unit that is mounted under the plastic of the console between the driver and the passenger seats. https://www.youtube.com/watch?v=l7fZx1sU4_8 a quick description of how such a module is used in a car alarm.