[tobster1911]

As an engineer supporting RFID and other RF devices I can speak from personal knowledge.

RFID tags can be both Read only or Read/Write. The amount of data they can store varies depending on the device. They can also come with encryption or be open to anyone.

RFID is unique because it is passive. There is no battery in the system. This means that it can be placed in a sealed system and have potentially infinite shelf life.

The distances involved vary with the technology. The old 125kHz is short range < 1ft in most cases. The newer 13.56MHz is mostly used for cyptology (probably what would be used in this case) and has range up to < 2ft. The latest development is in UHF (900MHz). This can have up to 15ft range.

As widebody said...There is no way to know if the tag is being read. Although I would put a money bet on the fact that there would be heavy encryption on the tag. It would not be a simple open read tag.

As far as being able to track (i.e. follow as they move from one location to another) someone with RFID in real time? Can't happen unless they have huge electronic toll both type machines everywhere you go.