"Evidence" is something that we can all see...or that can be presented to a court, or shown to qualified judges...etc.

"Well, there 'was' evidence, but 'they' are hiding it, and I can't ...ummm.. put my hands on it right now."

Right...even *IF* that is true, it-don't-count!....Except as apocryphal anecdotes to convince those who wish to be convinced. Remember the "Nuclear cold fusion on a desktop" a few years ago? Remember a dozen cases of "perpetual motion machines"? Remember...no, you wouldn't...the guy who claimed he could season lumber with no cracking? Remember the Oak Island "pirate treasure"?   Remember 'levitation' by Transcendental Meditationists..bouncing around on mats? Remember several groups 'finding' Noah's Ark? Remember the search for The Ark of the Covenant? (no, not Indiana Jones version). Some monks in Turkey were supposed to have it, but they won't allow anyone to see.
...for that matter, remember the X-ray glasses that would let you see thru girls clothes?

I could make a list 6 pages long...some serious, some silly.(except that people believed them).

Yeah, the govt. does test funny airplanes sometimes, and there are photos of Groom Lake and Area 51. I would not expect anyone to talk about it much if they want to keep their job.
   I had an Uncle who worked in San Diego making blueprints in the aircraft industry in the 1950s. The VTO planes had just been shown publicly (didn't work real well)..He told me he had known about them for years...and he said "If I told you what we're working on now, you wouldn't believe me!"
But with these things there is eventually proof and de-classification. Are there designs adapted from 'captured alien ships'? Or actual alien ships? *shrug*...I'll wait & see, 'cause the premises of how they GOT here are kinda vague.


Oh, by the way...remember how Peach pits were supposed to cure cancer?