It actually being aliens and/or a proper UFO is vanishingly unlikely. We know where our entire tech base has come from and have voluminous evidence of every part of the development and production process.
Even assuming that the alien technology is something that we can understand today (much less back in the 1950's or whatever), that tells us little to nothing about how to produce that item.
Take a modern CPU back to the 1950's and there exists basically zero chance (you have better odds of winning the lottery multiple times in a row) of anyone figuring out how to, even theoretically, produce such a chip. Much less build the required infrastructure.
Useful technology from an alien craft would be in the nature of truly novel/overlooked physics. Like if they had some anti-gravity device that wasn't actually that advanced but we had just never thought of. Anything involving materials science is basically a dead end, as are basically any electronics or IT related things.
Unless we want to also posit that they had a fairly substantial library on board and that we can access and translate it.
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Does alien life exist? Practically guaranteed.
Has it visited Sol? Doubtful.
Does the government have solid evidence of a visit to Earth/Sol? Vanishingly unlikely.
Does the US DoD sit on metric buttloads of game changing technologies for decades at a time? Absolutely.
The US National Security establishment throws more money at pie in the sky R&D every year than all but like five nations entire military budget, and has done so for like seventy plus years. There are a LOT of relatively novel things that are at least theoretically possible but haven't seen production use.
For example, negative index of refraction meta materials mean that actual visible cloaking of vehicles is possible. Even theoretically for RADAR as well. If anyone has figured out how to produce such meta materials at scale and has a use for them it is the US DoD. Truly invisible jets (except for the heat signature from the exhaust at least).
The US has also thrown more money into LASER research than anyone else for multiple decades and has, publicly, nothing at all to really show for it. And yet it is still billions per year being throw at the issue.
The US Navy has been funding fusion research since the 50's.
Assume a few secretive breakthroughs in Lasers, Fusion, and materials science and you might actually be able to put a viable fusion engine onto an air frame and give it a performance envelope that blows anything else out of the water. If you can do that and RADAR invisibility then you could make a stealthed space plane (probably). Make it a drone and you could be looking at a performance and maneuvering envelope that humans just couldn't survive.
That is also the kind of tech that you are going to sit on for a very long time. The US is already head and shoulders above everyone else in military power, it has no need to push that edge out even further in that manner. Sitting on it, on the other hand, gives an emergency capability if needed and secures you for another generation or two. And is not massively disrupting to the entire globe. And fusion on that scale would be a massive global disruptor.