Not people actually thinking they've discovered aliens and tried to cover it up. The US Government can't even keep hidden the fact that they're spying on everyone, how on Earth would they be able to hide the fact that Aliens have been visiting us? What's the chances aliens have only happened to land at Top-Secret military bases and not the middle of Times Square?
If Aliens had space travel technology we'd already know about them with the whole 'killing and conquering' thing, if they don't then there's no point in hiding their existence from us. Logic.