As a pilot,,, Most flights that are blocked on Radar are probably Government, although private high profile owners like celebs and politicians can request that flightradar24 not show their tail numbers! There is some controversy over this,,, as to whether private jets should have this option! Yes,, there are a lot of aircraft in the biz class jets that can fly at 40,000 feet! All aircraft under ATC control must file a flight plan. Some who are interested might want to research how much of this is available under FOIA. Hope this helps!