Well for those grownups, there is no contact in the picture yet. Anyway still just wait, observing, and see what happens?
What? There's contact/communication to answer the question "how are they so sure it's aliens?" but when the question is "what threats or opportunities do the aliens claim to present?" the contact/communication disappears again? These ETs really do travel quickly, at twice the speed of argument, which is far too fast for logic to keep up.
If I were the secret dictator of some powerful nation, and I were faced with the question of what to do with the reports of the type you listed, sightings of craft appearing and disappearing and moving at (believed-to-be-) impossible speeds and accelerations, here's what I would do:
- I'd want investigations of the possibility of these events being caused by technology we don't have, whether the reason we don't have it is because it's alien or because some purely earthly foreign power has made an unexpected breakthrough. So besides engaging espionage assets to find out as much as possible about what technology our allies and enemies have or are working on (which, of course, I'd be doing anyhow), I'd also be asking scientists and engineers, is there anything we can learn from these reports? Can this information help us in any current or possible development efforts of our own? To do this, I'd have to disclose the details of the encounters to those experts.
- While the scientists and engineers are in the room, I'd also tell them, "If these occurrences are due to malfunctions or shortcomings in our instruments and our observation methods, let's find out the causes of the false observations, and learn how to improve our instruments and observation methods to improve their reliability." And by the way, if the anomalous readings were due to spoofing or jamming or other interference with those instruments by someone else, especially one of the aforementioned purely earthly foreign powers, let's figure that out as well and learn how to prevent it.
- If the evidence of the use of alien technology turned out to be strong, I'd very much want to investigate and either confirm or rule out whether the aliens were assisting some other nation.
That all falls under "waiting and observing," but if anything of use turns up, it also counts as "preparing" (and if nothing turns up, there's either nothing to prepare for, or no knowledge of what to prepare for).
All of those measures involve disclosing of some information to some people, but none of them requires, or benefits from, "disclosing" wild-ass guesses to the general public. (There might be some espionage scenarios where a false or deceptive public disclosure—note that "true" can be deceptive too—might have some value in seeing how some opponent responds. But that's an unusual and mostly self-limiting case.)
Keep in mind, also, that not disclosing something doesn't mean saying nothing; it means saying all the same things you'd be saying if the something in question weren't the case. Which means that what authorities do and don't say about a secret topic is a very poor indicator of what's true about that secret topic. That, of course, is by design.
I guess waiting ie not prepping wouldnt matter so much. Not much point in preparing to fight such technology. Emilys Cat indicated its pointless, wanted to be able to fight anyway…
I agree that preparing for interstellar warfare against aliens that are already visiting our planet in spacecraft would be pretty pointless. One problem is that interstellar spacecraft that can travel between nearby stars in less than many centuries are also, by nature, unstoppable world-ending weapons. You don't need the Death Star or the Doomsday Machine to wipe out a planet. Any space ship (even Millennium Falcon size, let alone Enterprise or Star Destroyer size) moving at a significant fraction of the speed of light will do the trick, if it's set on autopilot to crash into your planet.
So, you concentrate on developing your own technology, which you already want to do anyhow when you're in arms races with other nations. You disclose your various "sightings" and "encounters" and any physical evidence (crash wreckage and such) as needed, but only to the extent that doing so helps in that effort. If the encounters prove to not be helpful (i.e. they start looking more like instrument reading anomalies and optical illusions and crashes of other intelligence branches' secret projects instead of aliens) then you don't chase those leads pointlessly once you figure that out. You leak some stories publicly, some true and some false, to keep your enemies uncertain, just as you'd do if there actually were some aliens around and you knew about them.
What eventually makes the situation clear is not what you say or don't say (which, to emphasize the point, if you're keeping a secret should ideally be the same as if you weren't keeping that secret because it's not true). It's the lack, over periods of decades, of any discontinuities in world history or world technological progress. Actual involvement of aliens or their technology would cause such discontinuities. If they were around, it would be no harder to figure out than whether or not any Europeans did or didn't reach the New World around the fifteenth century AD. (Spoiler: they did.)
