All this is telling us what the criteria is to be listed as a missing person and go into the registry. It doesn't indicate what the circumstances are for the FBI getting involved. The above is the "definition" of a missing person.
They're the FBI - I'm sure they can get involved in whatever they feel warrants their attention. I believe the nearest field office is in Omaha, so I'd guess that group is fielding this. It's not like they're flying people in from Quantico.