Radio frequency identification (RFID) tags, we use / used them for inventory purposes and tracking location on equipment, vehicles etc. You can track tag movement worldwide with them to within a foot or closer, it is some seriously cool Spock.
When they scan the bar-code/RFID tag in a retail environment it is "SUPPOSED" to wipe/turn the tag off since you are paying for it. Machines aren't perfect, stuff happens, keeps folks in jobs.
Believe it or not. The last I dealt with this stuff, a passive RFID (no battery/signal) tag can be read up to 40 feet away, depending on equipment. An active tag (battery and sending signal) can be read from further.
Found out my keychain sets it off due to my RFID work key to get into our warehouse...
have occasionally set it off with large magnets used in concealed carry holsters as well
Also remember Walmart put out a big thing about No Guns In Stores...but they won't make
a big fuss about it, dependant on how well you've hidden it, and if there was any kind of incident.
Most security folks are perfectly happy with CC individuals as long as they're not dumb morons...
i.e. printing badly & scaring the Liberals.
Oh, and some California-made pistols & holsters also have hidden RFID's...
doesn't hurt to check your gear...