I was coming back from Lethbridge a few days ago and my radar detector went off but there was nothing in sight and it maybe took a full minute of driving to realize there was a police officer pulled over on the side of the road , I wasn't speeding (110 in hour maybe)
If my radar detector can pick up the signal from so far away then i'm assuming a cops use of radar is similar but if they can pick up something that is moving faster without seeing the car how do they know which car it is if there are multiple cars on that highway?