The idea is the radar boards turn on an LED as people approach. They get close, they see an LED go on - they've been detected. They back off, the LED goes off. They get close again, the LED turns on again. They move to a different spot. The same or a different LED turns on - detected again. They don't know if these are connected to an alarm system, a camera or something else.
The alternative is a bunch of flashing LEDs that flash all the time. Nothing changes if they get close. Don't need a whole lot of brains to figure out nothing happens.
Less than a buck a module, plus an LED and resistor. That's pretty cheap.