The military is a possibility, but I don’t know about “most likely” here.
Given that assumption, the answer to your questions are:
> Why military?
The assumption here is that the military goofed, didn't tell another branch about it (hence the USAF scrambled), and is avoiding either embarrasment or needs to keep the deployment of the craft as secret as possible. This probably happens on a regular basis (though if we have data on how often that happened during the Cold war, that'd be useful), and explains why USAF couldn't find anything.
> Not smugglers
I have never heard of smugglers having advanced aircraft that would be flying around Oregon; Seems pretty risky. Also, doesn't make a ton of sense as you presumably wouldn't be able to transport much with a high speed aircraft.
Definitely an interesting proposal, have you heard of smugglers having advanced aircraft?
> intelligence services
Possible; Which ones, the American Intelligence Services? If so, I believe the parent was lumping them in with 'military'.
Though it still seems more likely that this was just bad internal communication, mostly because I wouldn't expect a foreign spy plane to be ever be caught flying at high speeds; though it could be a generic attempt to test the US response. Be great if anyone has any data on how often that sort of thing happened during the Cold War.
> just an asshole
Seems very unlikely. What random assholes have access to advanced aircraft that can evade the USAF?
So under the assumption that this was an advanced aircraft, the most likely explanation is government, either the US's or a foreign intelligence service. Since presumably the US is in control of it's own airspace, the idea that this was the US itself seems the simplest explanation.
Intercepting an inbound russian is easy. He is comming towards you. An airliner moving away or past you at 200nm is exponentially more difficult.
Perhaps intentionally provocated a response from FAA, USAF, etc as a real-world test of the aircraft's stealth/evasion technology
Although Southwest flies 737s and the SWA pilot did report it was bigger...
Could be a routine ferry of Nightwatch/NEACP based on the same profile, but seems like NORAD and WADS would have caught that sooner. And kiboshed FOIA and not made public statements and all that. Nightwatch is operated by USAF, though, under high classification, while WADS reports up through the National Guard. Miscommunication seems at least plausible.