UFO Amnesty: Ex-Army Colonel John Alexander Seeks Amnesty For Military Who Witness UFOs [EXCLUSIVE]
"'The reason is because we all had very high-level security clearances,' he said. 'Just the mention of seeing something that you didn't know or you thought might be a UFO would be enough for a visit with the flight surgeon, if not the psychiatric staff at the base hospital.
'And that would be reason to pull your security clearance, at least temporarily, which means that you wouldn't be flying and that would be a black mark on an aviator's record. Seeing a UFO show up on your base could be perceived as [your] being unstable. At least as far as retribution or career penalties, if things could be lifted or suspended, that would probably help the process.'"
From above linked Amnesty article on UFO sightings by the military.