it's not the levels of tech, more that it's easy to get things mixed up, or confused with something else when there is (usually) a more Earth-bound understanding.
GPS wouldn't work in the 50's, the first satellite was Sputnik 1 in 1957 (I did google that date because I couldn't remember how late in the 50's it was) point being, our understanding and references of technology are only what we see. If we are kept 20 years behind, then military tech is 20 years ahead. In 1990, British TV had 4 channels, by 2010 we had netflix.
Don't even trust RADAR for anything. Seriously. It's so easily fooled (The wedge shape of stealth aircraft is to alter the image represented by the RADAR, to make it appear bigger, smaller, 100ft to the left etc. RADAR tech is from the 40's and has changed very little since and i would take any reports from the War or after the War with a huge pinch of salt. A lot of propaganda and mis-direction was used to keep the enemy in the dark about what tech we had.
There is another possibility; that a foreign government, be it China, NK or Russia, have advanced tech, and what we're seeing is those countries testing it out over foreign soil to gauge the reaction times and detection ability of America. For reference, Russia 'buzzes' Scotland a few times a year to see if we Brits can scramble our jets in time. Russia get a slap on the wrist and we get a training exercise out of it. So it is possible that these craft are Russian or NK in origin. Don't listen to the lies and propaganda of the Americans either, the Russians never have been behind the curve, technologically. Never.
Again, not to poo-poo anything you're saying but I think it's best to leave modern day sightings to one side for now.
Now we're getting interesting. Yeah, the cave drawings, paintings and sculptures of modern tech in ancient times is...fascinating. The antikythera mechanism is my favourite. Some of that stuff is yet to be explained. Not only how did they make it, or why, but why wasn't it put to use across the empire/society that it was found in?