Indeed, due to the moon's pristine environment, any modification of lunar surface features will remain preserved for eons - the lack of an atmosphere means features are not eroded away. Unless intelligent aliens came, saw and then covered their tracks, perhaps they left something as basic as a footprint for us to find.
Alien-hunting programs like the Search for Extraterrestrial Intelligence (SETI) have primarily focused on looking for alien transmissions being beamed around the universe, but the probability of success is extremely low. Other methods of alien hunting are therefore being considered and the moon has just become the logical "intelligent alien hunting ground."
If these hypothetical aliens are advanced enough to traverse the vast distances between the stars, and if they decided to pay the Earth-moon system a visit over the past few million years, they may have used the lunar surface as an ideal observation post. Logical, right?.
The idea that some kind of alien artifact may have been left behind then makes sense. This "artifact" could be a footprint, spacecraft or structure - the LRO can spot the Apollo landers and astronauts' preserved footprints from orbit (pictured top), so it stands to reason that we have the technology to carry out this proposed lunar hunt.