See my above post to Ara Pacis. When I said "make themselves known" I meant "communicate". Like when you walk into a room full of people, you're not hiding, but you haven't made yourself known until you speak up. Despite that, I still think they could blind us to their presence if they were sufficiently advanced. I just want to make clear that although I think they could hide, I don't think they would have to hide to study us. It's true that we may change our behavior if we know they are watching, but I don't think it would change that much. You're talking about 7 billion people.