The motion sensor used in the home security system is known as the pear motion sensor. PIR stands for passive infrared because they detect infrared energy from humans. The skin on your body is around 93 degrees F. and produces infrared energy at around 9 to 10 micrometers in wavelengths.
So home security sensors are designed to "take" energy, in the range, 8 to 12 micrometers. Infrared energy "lumps" electrons from the substrate and these are converted and reinforced into a signal that turns off your thief alarm. You can buy a sophisticated motion sensor online at https://www.ligman.com/motion-sensor-innovate-for-future/.
The reason is to detect movement, and not someone just stands still because it is designed to detect changes in infrared energy, not just energy itself. When someone moves on the pear sensor lane, their infrared energy issues change, and that is detected. Amazing, no?
If you think it's sophisticated, get this: it also only detects fast changes, as caused by someone walking past "beam". It is so it does not depart with slow temperature changes, such as a heated room during the day, or cools at night.
Infrared energy is a form of light, and the reason that home security sensors can cover wide roads is that they use a lens to spread a large area. Want to know how many motion sensor areas?
Stand in the corner of a room in your home, hold your head silent, and the number of areas you can see with your human eye about the area that can be "seen" by the home security sensor. Is it cool, or what?
By the way, you might be wondering why the motion sensor doesn't "leave" with movements from outside the house, like someone walking near the window. The reason is infrared energy does not pass through the glass wall.