[true confession]
Had a dream a number of years ago where I saw two alien spacecraft hovering over my street.
My reaction, in the dream, was to sing the last verse of Styx’ “Come Sail Away” at the top of my lungs, in the middle of the street.
[/true confession]
So it would seem it would be rather difficult to convince what you saw to anyone, ie, chances are no one will believe you even if you, hypothetically… yes hypothetically, happened to see a real deal?
If, hypothetically, some alien beings wanted to spy on us, as long as they don’t park their mile wide spaceships over the major cities of the world (and other less spectacular public exposures), they would not have to be to concerned about accidentally being seen by a few random people? It is more than likely no one will believe anyway.
Fine - pretend I said “vanishingly improbable” - many orders of magnitude more so than me having a hallucination or being the victim of a detailed hoax. “beyond what humans can build” is one thing - FTL travel (what it would take to get aliens here) is a different animal.
I’d tell those closest to me but that’s all.
Where’s the “shit bricks” option?
I’d have to determine what would convince me I’m seeing anything far beyond known human technology. And I can’t think of a thing that would qualify it as an undeniable alien spacecraft.
I’d shake my head, blink my eyes, wonder “WTF was that?” and carry on to school.
Which is exactly what I did when I was about 17 years old and walking to high school. About a block away from my school I saw what looked like a silver cigar-shaped object hovering way up in the sky. I looked at it, looked away, looked back - still there. I blinked, looked at the ground, back up at the sky - still there. I wondered if it was a plane, but I couldn’t see any wings or lights and like I said it wasn’t moving - I was looking at reference points to see if it was - and then suddenly it shot off like a rocket and vanished. I can’t remember if it left entrails (?) (those smoke-lines that jets make). It was a clear, sunny, winter day with few clouds.
(In retrospect, my city is next to an army base, so maybe it was something military, I don’t know. It was still weird as hell.)
holy moly.
I’d take a ton of pictures-then use all my available funds to take short positions in a bundle of stocks-then release the photos.
I figure I’d be a millionaire in a week.
Haha - thank you. “Entrails.” 
So you are saying if you can’t imagine some never imagined, let alone seen, technologies from the future or unknown alien civilization, no such technologies couldn’t possibly exist?
For those who wants to take pictures reread OP: “but you are left with no photo, video, witness, or physical proof” 
I’d vote for this.
I’d go to my doctor and question my medications, or lack thereof.
But how would I know that the strange thing I’m looking at is alien tech? There are lots of things I have never seen IRL that exist right now that are totally human inventions.
I might suspect that something isn’t Man made if it is really huge, say like the space craft in Independence Day, and they were so large that there is no way I’d be the olny person seeing it.
Then there is the fact that I have been under the care of several psychiatrist, plus medication and I currently live in housing for people with mental health issues. No one is going to take me seriously. 
If there was no way I could provide proof, I’d keep my mouth shut for all time. But I’d do my damnedest to get proof of some kind any way I could, and try to engage some agency with an interest in the matter.
The poll is flawed in that the question itself assumes my response. I would not automatically assume that what I saw was an alien spacecraft - even if I had no idea what else it could be. I would attempt to remember as well as I could every detail, writing down notes as soon as possible (since the OP assumes I am unable to take pictures or videos). Then I would post here and elsewhere a description of what I saw, without making any assumptions as to what cause it, and try to get help in figuring out just what it was.
Get out my cricket gun and take cover.
Is this a lack of imagination problem here? You can’t imagine some fantastically fantastical spacecraft that does maneuvers not even seen on Sci-Fi/superhero comic book pages, and maybe even throw in some gelatinous beings inside, for example, that would make you “shit bricks”?
But then… medicated is definitely a trump card here.
I remind you again that this is a hypothetical question.