Sitting here knowing we have a helicopter and a land rover on Mars it doesn't feel like it's entirely impossible for an alien craft to be here, looking at us. I can't make the leap to believe without doubt there is, or even to believe there probably is. But it is not impossible. We've already proved it's not, and we're not really all that smart.
Space is really big. The difference in effort between getting an oversized RC car to Mars and getting a huge supermaneuverable space-and-aircraft to the nearest habitable planet is about the same as walking down tree driveway to get your paper and… well, getting a huge supermaneuverable space-and-aircraft to the nearest habitable planet. Then doing it over and over just to play mind games with the primitive apes on t…
And Time is really long, and what we don't know is still far greater than what we do know.
I agree it's not plausible within the scope of what we know.