A previous employer wanted me to investigate the possibilities of implanted RFID in order to:
- Track employee bathroom breaks
- Track employee hours of work
- Ensure building security
(in that order)
Cards/Tags were not sufficient because he was certain the employees would cheat him by swapping their cards/tags amongst themselves.
Fingerprint "auth" could not be used for sanitary reasons, retina scanning was too slow.
Passive RFID can be read about 10-15 metres with good enough reader tech.