The Mysterious Three by William le Queux

Vera's very direct questions took me aback, though I had expected them sooner or later. "Who told me? " I said, echoing the words in order to gain time for thought, my arms still about her. "Oh, I'm sure I can't remember. I seem to have known it a long time."

"It can't have been such a _very_ long time," she answered, still looking at me in that queer way that made me feel uncomfortable. "Surely you must remember who told you. It is hardly the sort of thing one would be told every day -- or even twice in one's life, is it?"