[crooked-v]

For something to last a really long time, you'd want to have it at one of the Lagrangian points. Have it be big and reflective - say, a tremendous expanse of reflective cloth behind a relatively small probe - and anybody with a telescope and patience will eventually spot it as a weird "star". Something shiny at a Lagrangian point will suggest something weird (compared to the non-shinyness of objects collected at the Lagrangian points of other planets), making it an obvious target for investigation by space probes.

There are actually a number of probes at the Sun-Earth and Earth-Moon L2 points already [1], because it's a perfect spot for telescopes and observatories. I don't know if any of them are big enough to easily observe from Earth, though.

[1]: https://en.wikipedia.org/wiki/List_of_objects_at_Lagrangian_...

[alphydan]

Not any Lagrangian point will do. L1, L2, L3 are unstable so a small perturbation will send your satellite away. Only L4, L5 will do under certain conditions for the mass of the satellite [0]

[0] https://map.gsfc.nasa.gov/ContentMedia/lagrange.pdf