If they belong to any military including our own, think of how destabilizing it would be...one military in the world has technology that is light years ahead of anyone else..
They can penetrate another countries air space with impunity, they can fly at speeds that no military can deal with....and probably can deliver some type of destructive power that can’t be defended against....
It might cause a potential enemy to attack without warning trying to get in the first shot...
I know for a fact these things were around in 1996. So if there are human/earth based then this tech was around a long time ago. I’d think that kind of tech would have been actively used by now in one conflict or another.