Your post really helped me to sort through some thoughts I'd been having. I had actually opened up a new post window with these thoughts an hour before you did.
The problem with cases like this is that a lot of the facts can be interpreted and affected by the eye of the beholder. But this theory feels likely, doesn't it. Like, top two likely.
I've had this vision of how it could all play out. And if you head south from, well, anywhere in the country, you can hit Interstate 40 and travel across the US from coast to coast. It puts me in mind of a trip I made from Washington, through Oregon, CA and straight across 40 in three days. I've been toying with how long it would take in a bus, as I was driving at the time, and how often they'd stop. I wish we knew more about how long he had to remain in an area for the test to pick it up.
And again, more anecdotal musings from myself that I've never really discussed before is a trip I made from the east coast to west shortly before 9/11. I tend to be hyper aware of my surroundings and most of my trip there were three middle eastern men who were acting suspiciously (and bear in mind two things here: I was raised within a sect of the Muslim sphere and to me these men would be considered more comforting in any situation than suspicious, and there was zero weirdness pre 9/11 that I was personally aware of) ... they kept sitting away from each other at the airport yet they kept making eye contact and though they seemed to be traveling apart they kept lingering near each other as we milled about. It was just odd and I took note of it. I ended up seated next to one of the men and he was incredibly standofish and unfriendly to me even though I kept chatting with the woman on the other side of him and offering him magazines etc... it dawned on me that he was probably rather fundamental and I was beneath him, as a woman. But whatever. I've never been able to shake the image of these men nodding at each other and the moment when one of them came walking up behind us in the plane, looked down at the man next to me, who shook his head, and he kept on going.
After 9/11 happened I mentioned this to some friends and they said that there were many terrorists who flew to get a lay of the land, connections etc before they settled on the flights that they did. I don't know if this was true, but it would make a lot of sense. And it makes me wonder if I did indeed have the pleasure of sitting next to one of these men, and luckily my destination wasn't a target. It would make a lot of sense to me that there would be multiple men who were a part of things, assisted, and dropped off into obscurity. And even that one of them couldn't deal with what he was about to do and went as far away as he could and committed suicide.
Some of the hubs these men likely traveled through were Chicago, Dulles, Sea Tac, RDU... A flight from any of the first three could take you to RDU in a matter of hours, and then they could get on 40 and boom, instant cross country travel.