This is not new technology... I’ve had a Bearcat scanner in my truck for over 8 years that has the “Beartracker” feature. It works using the same exact technology as described in the article. Sets off an alarm signal and a flashing red light whenever a police car is in the area. Has slowed me down plenty of times before I got to a radar trap, thus saving me a ticket. Also, since it is completely “passive” technology, it is undetectable in states where radar detectors (a non-passive technology, and thus detectable) are illegal.
That supports 800 Mhz but analog only, right?
Radar Warning Receivers (passive) have been around for a long time. A basic 'radar' detector doesn't intentionally 'ping' out some signal to detect if radar is in the area. It listens for emitted radar signals from elsewhere. The better the radar detector, the more the ability it has to look for pulsed signals, normal police radar detector bands, and even when a laser is illuminating it, but it is not designed to 'emit' anything.
I think where the confusion comes in is in the design. Actually the receiver part where the incoming RF signal is down converted to a frequency more easily processed by digital means, for example. And this design can be quick and dirty or more sophisticated. Cheap designs may use detectors, (mixers) that have bad isolation between RF and LO (local oscillator signal that mixes with the RF to give the intermediate frequency - IF output).
So, in effect, the LO could be transmitted back out of the receiving antenna. Better and more expensive designs (more than 1 stage of conversion, for example) are much less prone to do this.