No, they hide in plain sight, most are Politicians.
Has the government covered up evidence of alien life?
Well, that's a fun topic. If there was any actual concrete evidence of a government coverup involving aliens, I'm positive that the news of it all would spread like wildfire. Not just a few websites and tv shows here and there claiming things. If you look back in history, the government has been caught red handed in coverups. Not aliens, but other things like scandals and what not. When the government was caught, the news spread fast. Everyone knew about it. It was the talk of the town for months. People even remember it in the future. Referring back to it even if it was before their time. It's also in history books. So, I'm not sure if that's a yes or no question. They may be, and have just not been caught yet. But if anyone actually had any big evidence on a cover up like that, it'd be out and we would have heard about it. (My theory)
That was way too long of an answer for that question...
There has been none to cover up.
Governments cover up many things for many reasons but little intergalactic warriors are not among them. They make great characters for books and movies, though.
The Earth is actually a giant space ship flying through space.