The flux follows the inverse square law, however, which means to detect if a rhino is tagged you'd have to get really close. Admittedly I don't know anything about the mechanics of rhino poaching, but considering how aggressive rhinos are I imagine poachers would prefer to shoot them from a decent range for their own safety.

No one is talking about scanning live rhinos. Even in the best case scenario, this is to catch poachers long after the rhinos are dead.