Considering the tendency for large percentages of the population to behave in a highly emotional manner (hysteria/violence) there is an argument to be made that government would be forbidden by overriding concerns of "domestic tranquility" from disclosing evidence of non-human intelligence outside Terra.
There is some evidence that a significant number of people might flip out if they thought the government had presented evidence of such a thing. They might be alarmed at the new paradigm or suspicious that a conspiracy was afoot to hoax/mislead the public - and become dysfunctional (experience emotional breakdown) or violent.
I suspect that if such evidence existed, the government would naturally conceal it for the above-stated reason and others.
I know some individuals who believe they saw a genuine extraterrestrial craft passing very close overhead - but they were highly intoxicated.