Possibly expansion and contraction of the tiny metal strips in the reeds as the temperature changes could cause false alarms. Another possibility is that the magnet and switch are too far apart. Usually, the gap shouldn't be more than 1cm or whatever the manufacturers recommend. If the gap is too great, the switch may be only barely held closed and any temperature changes or vibration could shake it open. The reed may also be faulty or low quality. I've had to replace lots of contacts, mostly on windows that were only rarely opened. So they seem to "stick" if not "exercised"