It'd make more sense to track people via the rfid chips in their credit cards. Obviously you get an update when the card is used, but it could also be used for blanket surveillance given the right setup. Essentially pinging the card.

I'm convinced that this was the intent behind the whole chip on card thing before smart phones came about.

While technically possible, RFID is an absolute dogshit method of tracking. You'd only get location updates when the target is wearing the tagged clothing and only if they're in proximity to a scanner you control.

My question is more of a "why" rather than a "how".