When I was stationed a few years ago at a military base in south-central NM, there were quite a few "experimental aircraft". For instance, I can recall one time sitting outside on the back patio at night, in the dark, stargazing, and having 'something' go by overhead that was completely silent and had NO visible lights. Had to have been ~200 feet above the ground, V-shaped and absolutely NO sound. It happened a dozen or so times while I was there.

So I find your theory plausible, that its quite possible these UFOs are (hypersonic) stealth aircraft. Too bad we will never know or be told the truth...