Aliens Meeting Directly
The first thought that comes to mind, "Would we be able to recognize an alien?" Seriously, there maybe a need to train ourselves to recognize them. Maybe our senses need to be finer tuned. Or for that matter, maybe they have no intention of letting us know they are here.
I do believe that a definition of "Alien", in whatever description, will be incomplete. Something foreign to us is probably more accurate. Something unexpected or unwanted, would be a more refining definition.
But as humans, we tend to fear the unknown. As long as we fail at understanding, then Aliens will continue to be something that resides in the unknown and will continue to instill fear and distrust.