On flightradar24 I saw a plane flying as high as 59,700 ft at a bit more than 40 kn (pretty slow to fly at such altitude) over north-east Missouri at 1:18 am CST. Fr24 didn't tell what plane that was and I guess it's an experimental plane or drone. It could also have been an espionage balloon, but it was portrayed as a winged aircraft. Does anyone know or have an idea what that could be?
A high-altitude balloon, identifier HBAL577, registration N257TH.
(FR24 may be storing "altitude" as an unsigned 16-bit integer; notice how the altitude seems to wrap around back to zero in the vicinity of 65,500 feet.)
More information available at this FR24 blog post. Notice how they say "Because the balloons do not necessarily transmit their type, some may appear as aircraft icons until we are able to update our database."
The FAA's N-number Inquiry site shows that "N257TH" has been reserved by Raven Aerostar, who formerly collaborated with Google on Project Loon (although it is not actually shown as assigned to any aircraft, which is why FR24 did not know to show it with a balloon icon). This article from The Drive notes that Project Loon was shut down in January 2021 but Raven Aerostar continues to develop its balloon technology for military applications.