The bottom of the sensor has three metallic contacts. The two smaller contacts are used to detect moisture and if your adhesive device covers these or lifts them above the surface too high, then they may not detect water or moisture soon enough for you to react to the alert.
The larger metal contact is used to measure vibration and must make full contact with the surface that it's mounted to in order to correctly detect different vibrations such as a door knock vs. the door opening and closing.
We do not recommend the use of Velcro or magnets in these cases because it may lift the sensor too high above the surface. A thin, double-sided adhesive is best and Notion provides custom-fitting pads for this purpose.