sensors falling off and setting off alarm

@ kevdia2 suggest you do a google site search and review comments already posted on this subject.  If you do have to reinstall fallen sensors, you have to use new strips. 3M command strips are excellent, including the newer velcro types.  Note the surface area must be clean, flat to work properly. And when in doubt, use the screws provided.

If you have them on any surfaces that are rough, like rough wood, the 3M strips won't stick to those worth a darn.  Surfaces have to be super smooth, clean of any grease especially, and you have to heed the instructions that tell you to press down hard for 30 seconds, to the the strips to bond with the surface well.  If you do all that, they should last for quite a few years before they decal and lose their hold.  Make sure none of the sensors get hit by direct sunlight, that will degrade the strips quickly.

For any surfaces that are not suitable use those screws.  Take the strips off first before installing with screws otherwise the strip gums up the screw threads and makes it impossible to drive them through the strips.