Question:How does a passive infra sensor detect the presence of a person?
The sensor detects the body heat of the person.
Everything around us and all of our bodies emits small amounts of infrared radiation all of the time.
Human bodies are usually hotter than the surroundings, so we emit more infrared radiation than the objects around us.
The passive infrared sensor therefore detects an increased amount of infrared radiation if a person is near it.
So it is not that the room is not hot, or not emitting any radiation, it is that the person emits more infrared radiation because body temperature is 37 oC and room temperature is only 20oC.
NB. Infrared 'Heat' sensors have nothing to do with detecting movement.