> One thing I worry about is a limited access tag - one which only > responds when tickled with the right stimulus. Such a tag could be > undetectable to the taggee.
A nonlinear junction detector could be a reliable way to find it. You won't find a tag hidden in an electronics device (NLJDs are handy to find semiconductor junctions in general, so you'd get too many false positives.) You could find it reliably in eg. a t-shirt or a banknote, where there is no electronics supposed to be. Besides, if such technology will be popular enough, the readers will have to be widely available on the open market. If there will be a code specific for each class of the readers, it will be possible to eavesdrop it in the vicinity of the given reader. Pondering the banknotes. The muggers will never have to follow their victim from a bankomat through a dark park anymore. They will just wait there, remotely scanning the wallets of potential victims, picking the ones stupid enough to carry more money without using a wire-mesh purse. The cops could use it too, for picking the persons with "suspicious amount of cash" - see the seizure of cash under asset forfeiture laws, dubbed "War on Cash". Just musing...
