How to not activate metal detector?

Stores do not use metal detectors, airports, etc. do - this is why your purse goes thru an x-ray machine at the airport instead of thru the metal detector with you. Shops use RFID (radio frequency identification) tags on merchandise with RFID detectors at the door... these tags can be almost perfectly flat and hidden in merchandise, and are deactivated by store personnel when you buy an item. Perhaps the tag buried somewhere in your purse was never de-activated when you bought it?