Well, at the true extreme that would literally be the case--if you are an archeologist and find The Gospel of Anne that no one else alive has ever read, and use it in your fanfic without citation, then that's plagiarism without question.

Not quite as extreme, but still pretty clearly plagiarism for me, I own a couple books that were vanity published by a local community figure. There's no way anyone on my flist not there because we went to high school together would have read the books--and since I went to a private school some towns over, I have strong doubts even then.

A potboiler in a genre not often read by fansis further into the grey area, but possibly not by much, a beloved work of children's literature may well be coming out the other side into "not plagiarism," etc.

Is that any clearer?