re: only speculation
Assuming our civilization and all signs of it on Earth disappeared in a magical puff of purple smoke, how long would it take a spacefaring civilization to find the artifacts WE left on other planets? Or in deep space?
Some are a lot more obvious than others. How hard would it be if you weren't looking for them?
The lunar landing sites would probably show up quite readily in any intensive orbital survey. I was also impressed by the photos from Martian orbit that showed the tracks of the rovers.