The Door and Window sensor is looking for a ¾ inch or greater separation between the transmitter and magnet for it to trigger an alarm. Check the following if your sensor has triggered a false alarm:
Ensure both the transmitter and magnet are still present on the door/window or frame
Be sure to verify when the door is closed that the arrows on the two pieces are perfectly aligned, within a thumb’s width and flush
Ensure there is no play (movement) in the door or window when it is closed
Ensure that the notches are lined up directly across from each other