A VG2 gun is a special device that police can use to identify the presence of a radar detector. It looks like a radar gun, but it does not detect speed. It detects radar detectors.
All radio receivers emit a small amount of radio waves. This property is inherent with radio receivers. The same is true of radar detectors, as they too are a radio receiver (microwave receiver). The VG2 gun looks for the radio wave leakage from radar detectors, and can thereby identify if a vehicle is using a radar detector.
Radar detectors are illegal in Virginia, The District of Columbia, and most parts of Canada. Radar detectors are also illegal for commercial drivers in all 50 states. If the above does not apply to you, then you need not worry about VG2.
Most modern radar detectors incorporate circuitry to be invisible to the VG2 gun, and are therefore "undetectable".
It is rare to get a false alert on VG2. Most false alerts occur when another radar detector is nearby, within about 5 car lengths.
Spectre is a newer model, made by a different manufacturer. It works in a similar manner to the VG2 gun.