Passive infrared sensor detection zone
PIR sensors work by focusing several narrow "zones" within the field of view onto a pyroelectric sensor. When a warm body moves into and out of these zones, it is the amplitude of the AC signal produced by the sensor that triggers a detection.

The effective sensitivity of the detector across its field of view depends on many factors in addition to an object's distance from the sensor, such as the size and temperature of the moving body, its speed and direction, etc.