Ahh... I haven't really bothered to take the time to read it just yet, but I'll say something you might want to think about.
How an alien looks like is quite unknown. For all we may know, some may exist in our eyes out of air. Humans are quite primitive, of what we know, our eyes cannot see everything, and not necessarily everything has to have a heat source. Animals, humans (practically the same) are warm-blooded creatures, therefore there are multiple ways to recognize it. A fish and reptiles (if I remember correctly) are cold-blooded creatures, and not as easy recognizable, but still are. Steel and rock/sand are materials that cannot be recognized by mere machinery. Air cannot be recognized either. We can breath it, but we cannot see it. What if a creature (if that is the right word then, that is) would exist out of all these four options at the same time?
Not to mention that it could also be the fact that our mind isn't able to possibly imagine how something may look like.
Or what others mentioned earlier, "intelligent aliens" is something that cannot be said with confidence either. Who says that those things think? Maybe they use a far more simple or complex way to decide and/or judge about things. Perhaps their memory is of a fish (2 seconds) and they have to live on basic instincts. Everything is possible.
Instead of bothering with aliens, my opinion is that the human mind should be developed more. Fact still remains that we only use a small percentage of our brain. What would happen if we managed to use all of it? Would we all go insane?
I believe it.
