How to create a dual zone microwave sensor
It is possible that the microwave system is very similar to that used in the older microwave police speed detector (radar gun). In your example, a low-power microwave signal is transmitted by a non-directional antenna. Any near-field disturbance is reflected back into the antenna, and is detected by a direct-conversion receiver (usually just a diode). Output from the detector is low-pass filtered to nearly DC. Near-field disturbances vary that DC voltage. Variations in that voltage signal something moving. Sensitivity is high inside the car, because mircrowaves are bouncing all around. Sensitivity outside the car is lower, because less microwaves leak out through plastic parts and windows.
Wiki page for "radar gun" goes into a bit more detail. In this case, the frequency of the receiver's output is proportional to vehicle velocity.