2- Today there are all sorts of navigation beacons in different shapes, sizes and colours, for example I can see a red/white striped one across the harbour from my window. Same with ancient stones, they're all different shapes and sizes..
back to aliens needing navigation beacons again eh? Have you not considered if they exist, that they simply have their sat navs IN THEIR CRAFT?
BTW what harbor do you live near? You say you can see it out the window?