Amateur plane watchers tracked an unmarked US jet flying above Scotland at 45,000 feet on June 25 last year, with the plane's ID associated with previous "rendition" flights. The Register suggests the plan was to grab Edward Snowden as he escaped to Russia.
The plane was flying at 45,000 feet, presumably to avoid having to report its presence to air traffic control, as if you're clever enough to get yourself that high you're not required to communicate your presence. But amateurs with radio receivers tracked the Gulfstream V's transponder signals, working out its location and following it from Washington DC into Europe.
The flight didn't make it all the way to Russia, instead parking itself at Copenhagen Airport. It may have just been delivering authentic US burgers to a wealthy ex-pat. We'll never know. [The Register]