A Sufficiently Advanced Alien
race that, well, isn't all too alien. In fact, they "come" from Earth, just like us humans, but their civilization is so much older and more advanced than ours that they have no trouble hiding from us (for whatever reason
Different from The Masquerade
in that the masquerade is the act of hiding (e.g. in Men In Black
extraterrestrials are walking among humans), whereas this is about the origin of the hiding species.
I'm using the term coined by John Keel for this concept but if anyone has a better title...