[Governor Dinwiddie]

If he left the United States, his passport was scanned by the TSA. Or if he took a domestic flight somewhere, that is known too. So the TSA has direct knowledge of his last known contact with their system. That would reveal the initial route of Halper's disappearance.

If there is no TSA record, then maybe it's time to suspect Arkancide.

Of course all major air transport hubs share information with the TSA too. I'm sure that President Trump has a rough idea of where Halper is.
 

[Seaplaner]

Credit card use?

Phone location?

Facial recog?

Friends, relatives, confederates?

Visitor log to Fort Marcy Park?