I think the film 'Close Encounters' has the right idea - if the world's governments contacted alien life, they would keep it from the people and look for a military / commercial angle. I prefer to think that alien visitors have no visible form at all, and move among us at will. It seems unlikely, though possible that visitors have come here. Maybe they communicate by smell, or colours, or are bacteria-sized, or maybe they took a look at a Robert Kilroy-Silk show and decided to try elsewhere - the possibilities are endless. If you do find out, tell us won't you?