open secret
Something that is supposedly clandestine but is in fact widely known, as in It's an open secret that both their children are adopted. This expression originated as the title of a Spanish play by Calderón, El Secreto a Voces ("The Noisy Secret"), which was translated by Carlo Gozzi into Italian as Il pubblico secreto (1769). In English the term came into general use during the 1800s.