Approach the motion sensor when it is inoperative. It may go inoperative in cycles or be controlled by a remote control system.
Line the edges of the viewing window with tape. The viewing window is a translucent plastic window facing the area the motion sensor is surveying.
Fill in all the gaps between the tape edges with tape. Make sure that the viewing window is completely covered. This will present the motion sensor with an unchanging view, and it will not trigger an alarm when it is activated.