Radio Frequency Security Tag

A local shop sold me a product yesterday - I have the receipt! - but they forgot to take off the security tag. The alarm didn't go off, though! I wondered if it worked by electromagnetic induction - that ferro-metallic strips inside disrupt a magnetic field created by the detectors at the door. I discounted the idea because any ferrous metals would set off the alarm regardless of whether it's a shop product. Also magnetic fields might interfere with portable computer storage devices. A bit of poking on the Internet found this http://www.explainthatstuff.com/rfid.html The implication is that the doorway detectors are radio frequency transmitters that won't harm anything. But the strips in the tag are aerials to pick up the signals and re-transmit them. The energy must come from the electromagnetic waves themselves. I'm not actually sure that the strips inside looked like aerials. Could it just work by reflection? But if so, we'd be back to the problem of metallic things in customers' pockets setting off the alarm.