The aXbo-sensor saves power and is designed in order to minimize electric smog. For this it was necessary to use purely mechanical motion sensors. Each sensor contains two motion sensors.
In every mechanical motion/tilt-sensors are two small balls (see picture), which already perceive the slightest movements.
These little balls cause marginal sounds that are barely audible - especially when you sleep - because the sensors are inside the noise-absorbing toweling wristbands.