What Have Your UFO Aliens Been Doing?

Assume for the purposes of this post that we believe (as I don't) that at least some UFOs involve alien technology. If so, when did this alien technology reach our solar system, and what can we infer it would do during its tenure here?

By "alien technology" I am indifferent about whether the technology is "manned". I assume that aliens are at least millions of years more advanced than humans. So I assume that aliens have long since traded biological substrates for technological ones, at least for expeditions like a mission to Sol. I also assume that they can imbue their expeditionary technology with alien intelligence and values.

We infer that the aliens aren't grabby, because 1) they haven't grabbed Sol's energy or material resources, and 2) they haven't grabbed the energy resources of our galaxy. Reasoning from the Fermi Paradox, we specifically assume that the aliens

• were the first technological intelligence to start expanding in our galaxy;
• are applying a Zoo Policy to not only our solar system but to nearly all of the exploitable resources of the galaxy;
• are somehow able to coordinate and enforce their Zoo Policy across thousands or tens of thousands of light-years.

Note that this model applies even if the aliens are a federation of civilizations with independent origins, though of course Zoo Policy enforcement then becomes more problematic. (If the first civilization are adamant Zookeepers, they have to ruthlessly enforce their policy on any new civilizations they allow to federate with them.)

We infer that if they forego the exploitation of our solar system's resources, it is because they value any local biosphere's evolutionary investments and achievements. So it's plausible that they would want to protect local biospheres from extinction events. Do we see evidence of this?

Venus and Mars became suddenly unfriendly to life roughly one and four billion years ago, respectively. Neither was due to impacts, and it's not obvious that protective aliens would have intervened in either case.

But it would be trivial for aliens to protect Earth from the impacts that have regularly confronted Earth with mass extinctions. (We ourselves are at most a century away from being able to do so.) So what recent catastrophic impacts should they have stopped if they were on duty at the time?

• 66Mya in the K-Pga extinction an object a few tens of km diameter took out the dinosaurs and all tetrapods >25kg. 
• 35Mya the E-Og extinction was perhaps caused by the Popigai impactor.
• 800Kya the enormous Australasian strewnfield was created by an impact whose crater has not yet been found. It's unclear why this event is not associated with any mass extinction or climate catastrophe.

That E-Og wasn't prevented suggests the aliens arrived too late to stop it. It would be very coincidental that the aliens arrived so recently that they could not stop the Australasian strewnfield event. (And no, the strewnfield is not because the aliens blasted the object to pieces. You prevent impacts but nudging the impactors, not by turning them into shotgun blasts.) With primates blossoming into tool use while also hovering at the edge of extinction, it seems irresponsible for zoo-keeping aliens to risk allowing that impact.  

Many UFO believers speculate that aliens arrived due to our atomic testing. These speculations are some combination of fantastical and scientifically illiterate. The physics of detection and travel just don't work. If aliens are here, they've likely been here for millions of years, but E-Og limits their tenure to <35My.

Aside from protecting local biospheres from catastrophic impacts, what else would the aliens be up to? 

Monitor the Zoo inhabitants? When you set up permanent zookeeping in a solar system, do your monitoring missions use large easily-observed craft with spaceship-like acceleration capabilities? Or do you instead use small, stealthy, camouflaged devices, to allow close-up and loitering observation? If I were hunting alien observation tech, I wouldn't look in the skies for spaceships. I'd instead look in the trees, for birds and insects that are suspiciously hard to catch. Again, we humans are less than a century away from creating a never-ending plague of privacy-invading disguised surveillance drones. No human in 100 years will be using a flying saucer to spy on anybody. So why would aliens use them? (cf. "teasing", below.)

Abduct the Zoo inhabitants? That of course depends on Zookeeper ethics. Zoo Policy doesn't tell us whether aliens value individuals like they value biospheres. If an alien values secrecy but is willing to kidnap and probe an individual human, that alien seems more likely to euthanize the test subject than to return it to spill the beans. So no flying saucers, and no returned abductees. 

Tease the Zoo inhabitants? Some say the aliens tease us with UFOs to test our reactions and prepare us for "Disclosure". But these are aliens with a monastic devotion to leaving the galaxy undisturbed in the eyes of anyone with optical and radio telescopes. So why would they reveal themselves only a few decades after we first noticed their millions of years of self-denial?

Protect Zoo inhabitants from each other? If Mars has remnants of independent life, the aliens would know it and might very well want to protect it from Earth. There have been a lot of failed Mars missions, but Elon Musk hasn't gotten the memo. Shouldn't we expect the aliens to protect Mars? If not now, when? (In Arthur Clarke's 2010, the Monolith aliens told humanity to leave Europa alone.)

Protect Zoo inhabitants from themselves? If you think nuclear weapons give humanity the ability to extinct itself, you might expect the aliens to disarm us. Note that through most of the 1960s, a large fraction of U.S. Polaris missile warheads were inoperable. And there are claims that UFOs have deactivated nukes. So yes, if you believe in alien flying saucers, you probably believe that aliens have the ability to deactivate most nukes if launched. (But aliens surely know that nuclear war could not extinct H. sapiens, and in fact would set us back a century at most. As horrible as that prospect would be to us, the Zookeepers might consider nuclear war to be growing pains -- a minor incident in one of the zoo cages.)

Cooperate with the Zoo inhabitants? Most UFO believers say aliens are secretly cooperating with human governments. Such behavior would be strongly at odds with the Zoo Hypothesis. Recall that ZH says the galaxy looks uninhabited only because the Zookeepers are maniacally committed to not letting knowledge of their presence interfere with the advanced development of other biospheres. So again: why bother keeping the galaxy looking uninhabited if you're going to start working with the local primates around the same time that a Fermi notices you're absent? Oh sure, they're cooperating "secretly", but millions of UFO believers claim it's easy to see through the "secret". If these advanced aliens can't fool the History Channel, they're not very advanced.

Bottom line: most alien-UFO claims and theories are nearly impossible to square with what we can infer from 1) how long aliens would have to have been here, and 2) what they would and would not do once here.