The problem is, other devices that are not radar are also operating on radar frequencies. A detecting radio must respond to them, too. Every response indicates a threat, a bogey. Only your judgment can distinguish actual radar warnings from the non-radar alarms. Let's start with the basics:

• X band: A catch-all band, still used regularly in some areas by traffic radar, but heavily populated by sensors for supermarket automatic doors and other nuisance signals. In shopping areas, expect door sensors. But know the territory. Unless you’re sure that X band is not used locally for radar, stay alert until you’ve identified the bogey.
• K band: Maybe radar, maybe not. Supermarket door sensors operating on K have recently begun corrupting this formerly reliable warning of radar. Another non-radar source—cheapie radar detectors that pollute by transmitting on K.
  Here are a few clues for spotting offending radar detectors. You may get a brief K warning just as you meet an oncoming car. Or a lingering K, nearly constant strength, as you move with traffic. Big hint: a direction change on the Radar Locator as you pass another car. Look for a detector in the windshield. But stay alert until you know for sure.
• Ka band: Watch out! Most of the new-tech radars operate on Ka. Expect some contamination from cheapie radar detectors, just as with K (clues above also apply to Ka). Do not dismiss Ka alerts until you’ve positively IDed the source.

Check the Bogey Counter, because many non-radar devices occur in multiples. For example, most microwave door sensors have at least two transmitters (for In and Out). Often such an installation will have multiple doors too, so there will be many transmitters. When you see two or more on the Bogey Counter, and particularly when you see it counting up quickly to four or more, you've likely found a nest of door sensors.

Burglar-alarm microwave sensors are often multiples too, because a single transmitter is not enough to safeguard an entire building. But microwaves from alarms are less likely to leak out of buildings. So alarms may appear singly or in low multiples.