We are hopelessly looking for aliens visiting planet earth scanning for all kinds of waves and activity.
I find the idea you would need energy to re-locate mass very primitive. Some even assume it would require large amounts of it. If it does require detectable energy we would at best see them leave. Traveling here still wouldn't have to show anything.
I think microbes, ants and nanotech make it very clear to us how large an alien ship could be. Large ships are only interesting for galaxy size warfare, terraforming and centralised manufacturing. If we look at our own history, any science fiction book, film or game we see scouting is not done in enormous vessels. Just like bird watcher's go virtually invisible to the bird.
What it means is that nanotech can help us discover much more about the galaxy, more information in it's turn means there will be more stuff that is naturally interesting. Using this technology we will grow an understanding of how invisible the bird watcher is when viewed from our nest.
Both knowing there is and knowing there isn't any life nearby in the galaxy would be meaningful to our philosophy. It would put new light upon- therefor give meaning to-our existence. At the moment it's much like we are just sitting here like a bunch of idiots. haha
If we discover we are alone in the universe we automatically secure our survival in the process. If we are the only life that ever existed we should take better care of ourselves. If we are just another species it will be interesting to see those Klingon's and pleiadians. http://video.google.com/videoplay?docid=-8949469271181885482http://video.google.com/videoplay?docid=-3064984200803032304