re: the dreaded X - one more datapoint

There seems to be some algorithm that can trigger this, beyond checking "the wrong boxes" that require a visit to secondary. When I got to the secondary agent, I got a "oh, you're a code something-or-another" you'll have to wait for the agent in charge.

No idea, but have used it several times since with no X. He just asked where I'd been, who I was meeting with, etc etc. Seems to be something triggered by which airline/location pre-reported you flying to the U.S.

Frankly, if the system does do that, points to them for actually systematically recognizing unusual travel patterns.