Maybe we already did that in a previous civilization, but lost the ability to recognize the beacons.

Isn't the moon protected even less from solar flares? How deep would you need yo build something on the moon for it to be protected, compared to on earth?

Gravity is a pretty reliable signal; doesn't decay much on million-years scale, and doesn't get obscure much either.

Not to imply those were from unnatural processes, more to indicate that basic spacefaring technology implies ability to detect such: https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)