I've seen a so-called black triangle three times.

Once in '97.

Twice this year. I suspect flying to and leaving a supposedly largely deserted air strip in another country that once served for U.S. during Nam.
Though this airstrip does have 24/7 security.

I've worked in aerospace: civilian, military, space (man-rated space vehicles).

My conclusion, thus far, is this:

Probably a lighter than air craft.
Due to the the zero sound factor it could well be scooting along with ion propulsion (developed for deep space travel... it could be adapted for a lighter than air craft in our atmosphere).

That's all I have.
I do know that what I've see is/are, "one of ours," due to the circumstances surrounding the '97 encounter... As well as this year.