Motion sensors in home automation frequently activate when nobody is present and fail to activate when somebody is. The detection method explains both behaviours.
The sensor detects heat changes, not motion
A passive infrared sensor measures infrared radiation across a segmented field of view and reports when the reading in one segment changes relative to another.
A warm body crossing between segments produces that change, which the sensor interprets as movement.
Nothing about the mechanism identifies a person, so anything that alters the thermal pattern produces the same signal.
Stationary occupants disappear
Because detection depends on change, a person sitting still stops registering once the thermal pattern stabilises.
This is the familiar failure where lights switch off on someone reading or working quietly, and waving an arm restores them.
Timeout periods are the usual workaround, which trades responsiveness for the risk of leaving lights on in an empty room.
Several things resemble a person
Sunlight moving across a floor, a heating vent starting, a radiator warming and a pet crossing the field all change the thermal pattern.
Reflective surfaces can direct heat sources into the sensor's view from angles that are not obvious during installation.
This is why placement away from windows, vents and heat sources resolves more false triggers than any sensitivity adjustment.
Mounting height and angle matter for the same reason, since a sensor aimed across a room sees a different set of heat sources than one aimed downward at a doorway.
Ambient temperature affects sensitivity
Detection depends on the contrast between body heat and the surroundings, so a warm room reduces that contrast and makes detection less reliable.
The same sensor that performs well in winter can become noticeably less responsive during hot weather.
Sensors that report their own temperature allow automations to compensate, though most consumer devices offer no such adjustment.
Presence sensing works differently
Radar-based sensors detect the small movements of breathing and minor shifts, so they register a stationary person that an infrared sensor loses.
They require more power and are usually mains-connected, and their sensitivity to any moving object means fans, curtains and pets need careful configuration.
Combining both types, using presence detection to hold a state that motion detection triggered, produces more reliable results than either sensor alone.
The infrared sensor is fast and cheap at deciding that someone arrived, while the radar sensor is better at deciding that someone is still there, which are genuinely different questions.