Moving Radar - Limited Range/Warning
In their efforts to increase revenue and "trap" motorists with radar detectors, the police use "instant on" more frequently than in the past. With instant on, the officer leaves the gun in a standby mode and there is nothing to detect until the officer physically pulls the trigger to clock the speed of the target vehicle. If you are the leader of the pack or the only one on the road and they tag you with instant on, there may be no advance warning at all. With instant on, you are counting on them tagging someone ahead of you. This is why detectors with the longest range are important. You may hear your detector giving brief, weak bursts as an alert that might seem to be a false signal. This is a perfect example of instant on radar being used ahead of you, and should be a signal for caution.
The other possibility involves "mobile radar" and instant on. So radar can be used in a moving patrol car, they use a separate, low powered radar gun underneath the patrol car aimed at the road to clock the speed of the patrol car.
This is a very weak signal and is difficult to detect because it is aimed down at the road instead of in the direction of oncoming traffic. When the patrol car wants to clock the speed of a target vehicle, they use the stronger radar gun you may see mounted on the dash.