There are many sources of radar that can trigger a radar detector. A police radar gun is just one of them.
Examples of other sources of radar include automatic door openers in drugstores and grocery stores, speed signs on the side of the road, as well as many cars and semis with blind spot monitoring (BSM) systems. These BSM and collision avoidance systems often operate on K band, are becoming especially common, and can be very tough to filter out.
Higher end and newer detectors have better false alert filtering and more types of filters to deal with different sources of false alerts.