In their simplest form, short range radar sensors are excellent motion detectors. They provide very fine sensitivity for motion while providing a wide viewing angle (70-80˚) and far detection range (10's m). OmniPreSense recently released an API update for its OPS241 and OPS242 radar sensors which enables a simple interrupt pin with filtering for objects. An example use case is detecting people walking on a sidewalk but not the cars moving nearby.
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/blog/show/simple-motion-detection-with-object-filtering