I'll throw out a theory and you tell me if I'm correct:
A Radar Detector is a lot like a police scanner in "search" mode: it has a computer-controlled radio receiver that can rapidly step through an entire range of frequencies, pausing briefly on each one to see if there is an RF carrier present. If there is, it pauses briefly on that frequency and does some additional analysis; if it determines that this is, in fact, a real radar signal, it begins sounding an alert, then continues scanning.
Assuming the above is correct, do we have any idea as to what the step size is (i.e. the difference in frequency between one hop and the next), how long it takes to tune to a new frequency, how long the dwell time is on each one, etc.? I read one estimate which said all three police radar bands could be scanned in under one second. Compared to a police scanner that can do maybe 40 frequencies per second that's amazingly fast! How do they manage it?