Additionally, useful surface structures probably would be made in part of lightweight materials transported from Earth; a useful base would likely be largely subsurface. There's no particular reason why an entrance to such a base need be durable or monumental, or even directly visible with human eyes (or artificial aids thereto) from many kilometres above (let alone from Earth).

Apart from radio beacons and similar navigational aids, there's good reason to think a predecessor terrestrial spacefaring civilization would see very differently from mammals closely related to humans.

(Along those lines, one can imagine a predecessor Earth civilization whose leading species is comfortable in a temperature or pressure that humans could not tolerate, and that their comfortable working conditions would drive the location and composition of a lunar base for them. Even our own species had (and has) mountain-dwelling civilizations in environments challenging to subtropical coastal people. If they had been in the Space Race, mountain-dwellers might have had some advantages in needing to haul around less atmosphere (being comfortable with a higher cabin altitude), and they're the same species as the first Americans and Soviets in space.)