Reminds me of a flight I was on in the late 90s... I was flying jumpseat on an American MD80 observing flight crew ops and listening in on ATC. At some point over Nevada, the Center controller called traffic at our 12 o’clock (can’t recall the stated range), but did not mention an altitude. I remember the pilot and co-pilot briefly glancing at each other and then responding to ATC inquiring about an altitude for the traffic. ATC replies... “uh, its a bit above you.” The pilots leaned forward and looked upward, as did I (MD80s had the little windows up high)... now we were at FL370 (37,000ft) IIRC, and way up above there was a tiny spec that had overtaken us and was quickly pulling away. I’ve seen one other aircraft in my life that looked like a spec moving that fast... and that was a Mach 2 flyby of an SR71 at the Edwards AFB Open House in 1999... that time I was on the ground (rather than traveling M .78 in the same direction). No idea what it was... didn’t get a good enough look, but it was fast, and ATC was fully aware of it... and knew it was above 60,000 ft as well since the controller was clearly not concerned about a possible collision)
Best guess?
Let's face it a plane that can go fast enough that if it were just crossing the Pacific coast and heading for the east coast when it got a "land on the Pacific coast", order, would be over Kansas before it got turned around, can be hard to identify under even good conditions - the close the encounter, the harder the identification...