This doesn't have to be geopolitical or cryptography-related. He could have committed some ordinary crime -- say, he had a hobby stealing Legos -- then someone at the university said "Why is this closet in your lab stuffed with hundreds of thousands of dollars worth of Legos?" University isn't the police, so he's not arrested -- he's just put on leave until they figure out WTF is going on with this bizarre situation. Removing contact information might be SOP for any university employee placed on leave for this type of investigation.
(Note, I have absolutely zero reason to believe this man is stealing Legos. This is just a hypothetical example of a slightly bizarre but not entirely implausible crime that has nothing to do with politics, geopolitics or computer science.)
Of course in this situation, being placed on leave by the university has thoroughly tipped him off that the cat is out of the bag, and it's only a matter of time until he's arrested. So he skips town before the police get involved.
Why's the FBI involved?
- (1) Maybe it's a FBI relevant matter (e.g. there's a whole Lego theft ring)
- (2) If he told his family where he went, the court system could force them to reveal that information to the police, under threat of jail time. It'd be safer for his family if he didn't tell them where he's going or why. So he ends up as a missing person, which triggers FBI involvement.
- (3) Maybe the FBI is just as confused as we are. They get involved it's thinking it might be national security related because of the international intrigue or cryptography research angles. They don't know it's merely an "ordinary" crime (or at least they didn't when they decided to get involved -- maybe they've investigated enough to figure it out by now.)