I think there are at least two plausible explanations:

1. They are actually from Earth, hiding somewhere on the bottom of the ocean. Probably Von Neumann probes which arrived millions of years ago, and not actually living things. They check our status periodically and send information to their origin system.

2. We are living in a simulation, and they are spectating us in a god mode with a visible side-effect.

I like to imagine that if aliens have visited us, its for a kind of tourism or safari. Us seeing them is the pilot (some underpaid, overworked tour guide) making a mistake.

Travelling for many, many, many years just to see a bit of the planet from the window (and not even things like cities) is something I'd consider unlikely as the primary objective of visiting aliens.

Maybe they have enough of their own cities, and missing forests, large amounts of water floating at high altitude, whatever...

Teleport? Wormhole? Advanced tech that makes them pop up in our space? I doubt it. But not sure how else we have those things caught on military radars and pilot cameras that are not always explainable by optical phenomenon related to orbital objects or natural atmospheric phenomenon.

People thought there's no explanations for all the ghost ship reports until we figured out Fata Morgana, too. The space of mundane explanations is large even if unknown.

Except, they have video footage of radar screen spotting objects flying at high speeds and disappearing into the sea. Not sure that there is any other way to explain it than acknowledge that there are physical objects that pop up randomly, move at high speeds and disappear. What you make if it is up to your imagination, but you cannot dismiss as optical illusions. Maybe this planet is capable of producing physical objects that fly at high speed and then disappear into the sea. Some kind if illusive sea creatures that pop up to say hello then disappear?

> there's very few objectives they can be achieving with big ships flying around here for decades

The problem with this argument is that we wouldn't know in the first place if they were also using these more covert means. At some level, we have to assume that if aliens exist and are making themselves visible to us, they're basically doing it for the lulz.

There has been secrecy for decades regarding UFOs/UAPs flying on Earth, I think it's fair to say this secrecy extends to outer space as well.

there's very few objectives they can be achieving with big ships flying around here for decades (observation can be achieved with tiny machines, communication can be more direct etc.)

They may be doing more than just observing.