Important to note: there were no charge for him for all that time. That means they had very weak case. They might be specifically waiting for him to do at least that: just look how they accent that he "bought red flag last minute ticket" as if he knew he did something wrong. This also supports the idea that they had no proof of him doing what amounts to criminal industrial espionage (passing data to another company.)

Another standing out fact is that he was arrested minutes before boarding (he passed the border.) If he would be on exit control list, he would never be allowed pass the border. It means he was not nor on exit control list (people on it include convicts on probation, tax debitors, persons against whom a restraining order was issued i.e. people under investigation of a crime,) nor on the on-the-run list of criminals (he would've been detained immediately)

Third, how FBI ever knew of him buying a ticket? US is not East Germany where all ticket sales are wired to STASI in real time.

My explanation: they did not have anything qualifying for a charge on him till that "last minute ticket purchase" which added more substance to allegations of criminal conduct.

This is supported by the fact that he was detained "minutes before boarding." Probably, it was only the fact of him passing the border control that was visible to FBI, the moment they saw it, they came with a rushed arrest warrant.