Mellon, Elizondo, and most of the pilots who witness these things have said that they are so much more advanced than anything we can imagine. They are certainly not ours or our enemies'.
As an aside, new technologies do not emerge (poof!) out of a vacuum. They are part of a process of technological evolution and development. We would have noticed forerunner technologies and would be able to extrapolate from those to these.
“...pilots who witness these things have said that they are so much more advanced than anything we can imagine”
I wonder how any pilots that happen to catch a glimpse of an F-117 Nighthawk in 1981 would have described it?