It is not a stretch to believe radar absorbing tech is available to aliens, since we have developed it, along with 'light diversion' technology which sho9ws the background in the foreground of a cloaked object. What remains often is a 'cloud-like' formation which is caused by some ionization process associated with their gravity cancelling technologies. I was a field investigator with MUFON until I lost my mobility so I would point you to the MUFON files available to the public ... if the new owner of MUFON has allowed them to remain in the public domain; he has been using them to produce TV 'specials'. Since the forties there has been a growing campaign to discredit observers who are not the government workers (and many of them have been discredited, also, depending upon their level of clearance). That work is ongoing due to a perceived need to cover our own technologies. Weaponization of advanced tech is common, as you know,. Our fellow human sojourners are quite clever to 'back engineer' things shot down or found upon crashing, so expect the Chicoms and the Russkies to have much the same tech we have. And we have doubtless found much of what we have developed. When Ben Rich said there are things out in our deserts that are fifty years beyond what we can even imagine, he knew from whence he spoke. As to 'keeping secrets'. I'm 69, the youngest of three boys of my Father. It was not until after my father died that I discovered (this year in fact) that he worked at the Fort Belvoir area where CIA was established and entrenched until Langley opened. We always believed he worked for NATO!
By cloud formations I am not referring to lenticular clouds, which are a natural phenomenon.
How does a UFO crash?
What causes that?