Plane engine sounds are often beyond human perception at high altitude. But the sonic boom produced by every supersonic object, even at high altitude, is a known annoyance. Remember Concorde. No supersonic flights were allowed over populated land for noise issues.
A high altitude supersonic object would be noticed very often.
Yes, I hear them regularly as I live near an air force base. Sci Method brought up that airliners can be heard at 35k ft which is irrelevant since high altitude airplanes fly much higher. Concorde only flew to 55k ft. The U2 flew 70k ft and the military now has planes flying at 90k ft. plus. The higher up they are, the thinner/less air, and therefore the sonic boom (if any) is inaudible.
And on another note: These planes have high tech cameras on them, capable of zooming in on a golf ball from MILES high. So, there's your satellite photo....and Google Earth image.
OCCUM'S RAZOR
Satellite visible paths extend over 30 000 km (solid line)

Please give evidence of planes that can fly a sustained 90k ft altitude at the required speed, without refuelling (remember, the SR71 London-New York world record flight required an inflight refuelling).
Air density is not a sensible parameter for sonic booms. Distance and therefore altitude obviously are.
Here are typical overpressures:
Concorde, Mach 2@52k ft : 1.94 pounds
SR-71, Mach 3@80k ft 0.9 pounds
F-104, Mach 1.9@48k ft 0.8 pounds
At 85k ft, a SR71 was still audible.
Can you also explain how a plane is able to fly unnoticed over every country, even hostile ones ?
Finally, according to this,
http://www.heavens-above.com/AllSats.aspx?lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT over your head, for a given day, you may see 50+ alleged spatial objects. This can be roughly estimated to a 200+ world total. This is a lot of classified planes flying simultaneously, a lot more for the estimated fleet (refuelling, reparing and so on), and a gigantic amount of supply chains spreading over the world (airfields, mechanics, atc...).
Again, please give some evidence of this extraordinary activity.
For you side note. Can you give some rough estimates for the number of planes needed to cover the entire earth ?