You might not realize it, but all objects radiate heat energy (including your coffee table); it's just that you can't see it because heat consists essentially of infrared waves, which are invisible to the human eye (exactly the same as your TV remote control). These waves can, however, be detected by electronic devices designed for such a purpose, such as the infrared receiver in your TV that detects the energy emitted by your remote control when the buttons are pressed.
You probably do realize, however, that living things such as us, our cat, and the mouse under the floorboards generate quite a bit of heat. Passive infrared motion sensors used in security systems and automatic lights are designed to detect this level of heat. The term passive is used because the sensors themselves do not radiate any energy for detection purposes—instead, they just detect the infrared radiation emitted by objects. This is notably different from devices such as ultrasonic...