If we ever encounter extraterrestrials, it is much more likely that they will be from our own Solar System.

It also possible that they will be from our own planet Earth.

Things we know for certain

0) Properties of our Solar System:

it is far away from other stars (as in: you’ll need not-yet-existing tech, massive resources, and at least decades to reach another star)
there is life in our Solar System, and has been for at least 4 bln years
there are several objects within the System that certainly have liquid water (Earth, Mars), and there are also many strong candidates: Europa, Moon, Ganymede, Ceres, Enceladus, Mimas, Charon, Vesta, Tethys, Rhea.
the aforementioned objects are 4-8 OOMs closer to Earth than Proxima Centauri.

1) Properties of life:

the known life needs liquid water
some of the life is highly resilient, capable of surviving extreme conditions, including months in space
the life independently evolved into intelligence several times, and (in at least one case) - into a technological civilization
intelligence is a big evolutionary advantage (as in: the smartest species dominates the planet).

So, we have a life-producing place, and it’s producing and trying to spread life for 4 bln years, and very close to it there is at least a dozen of places that likely can harbor life too, and the life has the predisposition for becoming intelligent.

This is a contrast to the rest of our galaxy, which is very far away, may or may not contain liquid water, and appears to be lifeless (so far).

A case for Earth

Earth is producing technological civilizations. There is at least one known case.

Maybe there were others in the past.

Even 1 million years is enough to erase almost all signs of a technological civilization. But terrestrial life exists for 4 billion years. This allows for many attempts.

One especially curious case is troodontids:

close relatives of birds (we know that birds like ravens have intelligence comparable to chimps, but running it on a much smaller brain)
some species had brains 30% larger than of ravens (22 cm³ vs 15 cm³) [Jerison, 2004]
they had relatively long arms and fingers capable of gripping, and also well-developed binocular vision
the preserved communal nests indicate a social species
they apparently died out around 66 mln years ago, long enough to erase almost all traces of a civ.

A case for the Moon

The Moon is especially interesting:

close enough to Earth for a relatively high-volume exchange of material ejected by meteorites. There are terrestrial meteorites found on the Moon (e.g. “Big Bertha”), and vice versa
around 3.5 bln years ago (after the emergence of life!) it did have liquid water on its surface, some of which came for Earth. And at the depth of around 50 meters, the temperature and the pressure are suitable for underground liquid water: https://en.wikipedia.org/wiki/Lunar_water#Liquid_water

Implications for SETI

Seems it’s a reasonable idea to prioritize the search near home, including the search for extraterrestrial technological civilizations.

There are two main directions:

check the water-harboring celestial bodies, with the Moon’s interior being the top priority.
check the deep past on Earth, especially in and around troodontids.

What we may expect to find

The default scenario is “not even unicellular life”.

But it is not the only realistic possibility. An incomplete list of more interesting options:

1) Signs of a past technological civ.

The Moon is the top priority, again. Close to Earth, almost no geological activity.

If those troodontids achieved space travel, we may be able to find their stuff still preserved.

2) An active spacefaring technological civ.

I see no reason to exclude such a possibility. At least one already exists in our Solar System, thus there may be more.

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A fictional civ in the oceans of Europa

To illustrate the key points, below I attempt to describe such a hypothetical civilization, and the possible implications of it.

If there is such a civ under the 30 km of ice of Europa, can we even detect it? Most likely not, even if they run powerful military radars etc.

Why don’t we see their activity within our Solar System?

There is an important asymmetry here. As we are not under 30 km of ice, everyone can hear us, including their sensors on Europa’s surface. So, they know about us, and take a (very reasonable) effort to hide from us.

How do they look like?

The Europeans are Aquatic species. Not too different from the Earth’s life. Perhaps distant relatives to our tardigrades, who managed to survive on an ejected asteroid from Earth. The aquatic environment requires taking a quite different path on the tech tree (e.g. prefer bio tech to metals / silicon / electricity / mechanics). They grow spaceships (perhaps around their own bodies, by extruding protective layers of something like chitin, like tardigrades do to survive in harsh environments).

How advanced are they?

It’s reasonable to assume that it’s much harder to develop a tech civ in an ocean (a lot of advanced tech may still require fire, electricity, etc.). And the lack of starry skies may postpone many scientific discoveries.

So, it’s not given that they’re more advanced than us, even if they manage to reach Earth.

It’s a lot easier to reach Earth from Europa than the other way around. Just fall towards the Sun, and then do a few maneuvers. Can be done even with our 1960s tech, maybe even earlier.

If we ever find such a ship, it may look like many layers of hard organic material, wrapped around a desiccated pilot in the center. If the ship falls into our ocean, the pilot may wake up, and execute their mission.

Unfortunately, finding such an organic ship during the 20th century is unlikely to bring any technological advances, with a possible exception of some interesting organic materials, perhaps of a kevlar variety. One need the 21st century bio tech to extract more.

How we may cooperate?

Unlike the typical scenario of an interstellar contact with advanced aliens, the domestic aliens may actually want to trade with us, and it may make economic sense.

Aquatic civs may desire refined metals and many other materials that are very hard to produce in an ocean. Also our strange digital computers (sufficiently isolated for underwater use).

They may offer some interesting organics, e.g. a universal cancer cure. Or highly efficient bio weaponry.

Aliens from our own Solar System