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A civilization capable of interstellar—let alone intergalactic—travel would have next to no conceivable reason to spend decades furtively surveying one temperate planet by tootling around in its atmosphere.
The one and only conceivable reason for them to perform flybys of Earth would be to scoop water and/or biomass to refuel and/or resupply.
The most exciting possibility here is that we're an interstellar gas station.