Window or door status shows "Not clear", why?

The following message is displayed on the dashboard under the door or window status: "Not clear"?

If a sensor has temporarily gone offline, it cannot be determined whether the door or the window is opened or closed. In this case it is theoretically possible that someone could open the door while a sensor is offline. That is why the message "Not clear" appears. Only when the sensor is back online and the next time the door or the window is opened, is the sensor status is declared.