If it really is aliens, would they just give us the secret to perfecting inter-planetary travel already, damnit!? It's probably just some spoiled alien teenagers in their parents' new luxury saucer, cruising around the solar system, panicking us simple life-forms.
Seriously though, it probably is just some secret aircraft prototype or something. I know people are going to say "Well if that's all it is, why are they being so secretive about it?", well maybe because it's a classified military prototype?