what's plausible to me is that the governments of the world gain no benefit by hiding footage of an alien being. they are just waiting for the right time to release info that doesn't give any clues about their technology (such as UFOs which could give a competitive military advantage by keeping the technology secret). If they allow footage of a real alien being to be released, then they'd probably say, "yes that looks real. too bad there is no alien technology in the video.. maybe the being came here through some portal"