Riiight yes, gotcha.
Hmm considering how big the VentureStar would have been, my off-the-cuff airship theory seems like it might make sense - at least if we're not assuming some sort of extremely secret completely noiseless new propulsion technology, which just doesn't seem like something they'd be testing in populated areas where everybody can see them.
Shit, could just be a lighter than air "disinfo craft" – purpose-built to make people go "holy fuck what the hell is that thing", to make it seem like the US is fielding some sort of completely insane near-magical technology.
Oh yeah I'd loooooove it if this was some sort of absolutely bonkers new propulsion tech – let alone FUCKING ALIENS AKJDSAJKDASJKA – but it just doesn't seem very likely. Keeping something like completely new physics secret for decades, eh, I don't see that happening.