So these radar motion sensors are actually quite impressive, the ones with just analog output only have 1 transistor, the rest of the circuit is formed by the pcb traces. Weird, but it works. They can be hacked (sawtooth wave for power) to give a distance reading instead of doppler. The signal processing is too much for an arduino, but it is doable.
The other ones actually use the same chip as the most common pir devices.