Right, because aliens obviously have a money economy on their home planet, they use scarce and valuable resources to make gear, their mission will avoid the design issues that required us to leave things on the moon, and they think of property exactly as we do.

Well, you really have to start by wondering about "more likely to" in a debate that discusses if we have been visited by aliens or not.
But consider that you are a traveler, and are going on a trip very, very, very far from home base. Then it would be likely that you need to be a bit careful about what objects you can bring with you. We can't just assume that all they need is a bit of energy and some soil to recreate lost equipment. So while "salary" was a joke, it would be very likely that they would care about their gear. How would you value gear if you went on a many years long trip to a place where there are no stores that can sell you new equipment? It is quite likely that a traveler comming tens or hundres or thousands of light years would favour a sustainable system where they keep track of resources consumed. So if leaving any artifacts, it would be most logical that the artifacts left would have been produced from material already available here on earth, and not wasting hard-to-replace materials brought from home-base. Are you of a different view?




