There's a difference between being unobserved and being unobservable. — Michael

There can be, sure. "Unobservable" is sometimes used to denote that something is unobservable in principle. But it's also often used to simply denote what we could call "unobserveds," where it was a contingent matter that they weren't observed on that occasion.

What would you call phenomena that we're not even aware of, so that there's no concept of it, etc.? — Terrapin Station

To get a fix on that we can turn to that famous non-idealist philosopher Donald Rumsfeld (I'm deliberately misusing the word 'idealist' here, but why not, it's a Thursday after all):

Those phenomena are the 'unknown unknowns'. By contrast I think that the unobservables referred to in the OP are, in Rumsfeldian terms, the 'known unknowns'.