2. Knowledge of the actual existence of real non terrestrial entities, bound secret by unwillingness to publicly acknowledge neither their existence nor the possible fact that Govts cannot "control" them.
Such acknowledgement risks both unacceptable loss of face and possible public/religious panic and critical destabilisation of our acceptance of civilisation as we know it. After all, we "rule" this planet, supremely confident in the knowledge that no other being has ever "tamed" us.
Now, imagine the psychological shock to our species if we no longer had that confidence ? That's why, imv, Govts won't ever spill the beans in full.
No need to imagine. Read up what happens to 'isolated' aboriginal cultures when they are contacted by the 'modern' world. Their world-view is destroyed, and their societies collapse. This happened a few times during WW II as the Allies scrambled all over the Pacific in their war with Japan, and a few more times in the years immediately following the war. So it's easy to see how the slightest sign of 'extra-terrestrial' civilization would send those same military men into a panic about preventing any knowledge of those events or attempts at further contact. They had seen with their own eyes how small, primitive cultures were destroyed by sudden contact with larger and more sophisticated cultures.