When radar detectors proved easily capable of sniffing out radar from miles away, radar gun manufacturers responded by producing instant-on radar. In the instant-on mode, the radar's transmitter is placed on hold, ready to fire but not yet producing a signal for detectors to hear. The officer waits until his target is very close and releases the radar from standby mode and then gets a speed reading within a second or so. In this situation, no detector offers much warning.
TIP - When traveling in unfamiliar highways always try to find a vehicle which is traveling at the same rate or faster than you are and keep several car lengths behind them. This way the other vehicle can "flush out the radar".