No battery needed for My Wyze Smart-ish Lock Alternate Approach

My neighbor Bill for whom I made the ‘Wyze Smart-ish Lock Another Approach’ called to tell me that the button magnet glued on the end of his lock’s deadbolt fell off about 10 days ago and he was preparing to glue it back on. Bill noticed that the contact sensor worked although the button magnet was no longer affixed to the end of the deadbolt and had been removed from the lock area. He wondered why it still worked. My thoughts were residual magnetic field or maybe mechanical shock from throwing the deadbolt to open or closed.
To see what Bill is talking about, view:
My Wyze Smart-ish Lock Another Approach