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Serious question: how could they already know about us?

Are you simply assuming they have advanced signal detection technologies that match their space fairing capability?

We can observe a lot more in the cosmos than we can physically reach. We are mapping out and tracking far more stars and planets than we could hope to reach for a long time. Even within our solar system, we have gained and will gain much knowledge of the outer planets from probes well before we set foot on them. I would expect this property to hold for other civilizations, it's just much easier to observe far away things than to transport complex life forms to them.

Perhaps they periodically send probes like Oumuamua that report on nearby systems.

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