There are many types of occupancy sensors. The three most common are PIR sensors (or passive infrared sensors), which detect heat energy, ultrasonic sensors, which emit high-frequency sound waves and measure how those sounds are returned, and time of flight sensors, which do the same thing but with infrared light. Meanwhile, microwave sensors emit microwaves to detect people, while acoustic sensors measure people-made noise.   Note that many sensors today can be 'hybrid' or 'dual technology'. This means they combine different types of sensor technology to prove a more fail-proof monitoring system.