An alien civilization which happens to be interested in monitoring the evolution of life in the inner solar system, will not just fly through it once and for all. It would more likely send a mothership that deploys mini-probes into bound orbits around the Sun. This deployment can be achieved by launching the mini-probes with a velocity partly opposite that of the mothership, so that their net speed relative to the Sun would be below the escape speed from the Solar system.
The mini-probes will likely exhibit non-gravitational acceleration without the telltale signature of comets, a cometary tail. Even the largest ground-based telescope as well as the Webb space telescope cannot resolve reasonable probe sizes. Our largest spacecraft, 10 meters in length, would occupy an angular size of order 10 micro-arcseconds at a distance comparable to the Earth-Sun separation. Even if the it emitted millimeter-wavelength radiation from its ends, our state-of-the-art Event Horizon Telescope with an aperture comparable to the entire size of the Earth, would be unable to resolve the probe.
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