We have all seen these small tags on items in stores that have high cost and are easy to conceal and walk out of the store with. Since these items are left on the items sold the cost must be small and they need to be simple to de-activate by the cashier when the item have been purchased. The technology turns out be be very clever and extremely low cost. I had always thought the tag was getting energized by a low frequency field and then transmitting. Quite surprising that this technology works as well as it does since these transmit and receive pairs are usually 4 or 5 feet apart in practice. I guess a closed foil lined bag or pocket is all you need to defeat an active tag.
“Anti-theft tags contain a tiny mechanical oscillator that is activated by a magnetic field. The detectors at a shop’s doorway can detect the oscillation magnetically. The tag can be activated and deactivated by magnetizing or demagnetizing it, respectively.”