In the episode "Monk and the Sleeping Suspect", a man currently in a coma is still able to kill people with bombs mailed to his victims. Monk figures out how he did it. The guy used a special glue to attach his bomb to the top of the mailbox where it would stay for months. Then, when the glue evaporated, the bomb would fall down where the postman could see it and deliver it to the victim. Then, a sensitive motion sensor would trigger the bomb as soon as the victim picked it up.

So why didn't the motion sensor set off the bomb as soon as it fell down in the mailbox, or while it was being processed at the post office?