They demonstrate technology so advanced that it's more plausibly from another more advanced civilization than from a nation or person that lives on the same planet.
So if we get visited by "aliens" there's an even chance it will be some runaway Von Neumann replicator seeds shot out from a nearby solar system, bent on converting our entire solar system to computronium. Or if we're lucky, some relatively benevolent replicator seeds merely intent on cataloging all the myriad of life that has arisen around the galaxy without actually intending to destroy said life.
There is just about zero chance than any recognizably biological (I prefer the term "squishy") life coming to visit us, just because sending any sizable chunk of matter across interstellar distances at any reasonable speed so so damn expensive, even for Kardashev type 2 civilizations.
Given the language issues of the time, perhaps it is Biblical that life was created elsewhere and our ancestors arrived on this particular lump of dirt as spores on a meteor. Perhaps the same on the aliens planet. That would make aliens cousins rather than ... independent test tube creations or whatever. I suspect that given proof of alien civilization this would be a fairly popular outlook among existing believers.
If an alien civilization was to find and visit us (by means other than microbes riding an asteroid...) then their technology is at least a few hundred years ahead of ours.
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