You mention a "milky st[r]eak." This sounds like it could be underfixing (possibly caused by overused fixer). Perhaps you could try fixing again with fresh fixer. Even if this is the cause of that problem, it wouldn't explain the flat-looking negatives.
I'd also add that just because you've sent film through airport X-ray machines without trouble before doesn't mean that you'll always be successful with this. You could have encountered a machine that was out of spec, or different airports might have different machines or specifications, or an airport might have changed their settings or procedures, or any of a number of other things could account for having no problems before but encountering X-ray fogging this time. That's not to say that I think the airport X-ray machine was the source of the problem; I just think you can't rule it out as a possibility.