The Great Wall of China

It is often said that the Great Wall of China can be seen from the moon. Not only that, but it is the only man made object that can be seen. 

In fact, it can't be seen. There is no single man made object that can be seen from the moon. Continents, let alone countries, are only just about distinguishable. 

The only man made things that can be seen, as a collective, are lights. You my have seen images of the Earth taken at night; if you haven't, then google it - it is really impressive. You can still make out the continents but that is only because of the outline the lights create. If the lights weren't there, then you wouldn't be able to see anything on the Earth.

So if aliens were to pass by the daylight side of Earth, even at a distance much closer than the moon, and had no way of picking up radio signals, then they would pass by us, completely unaware of our existence.