open secret, an

Something that is widely known, although it is not supposed to be. Oh please, everyone knew he was the real leader of the department—that was like an open secret.

something that is supposed to be known only by a few people but is known in fact to a great many people. Their engagement is an open secret. Only their friends are supposed to know, but in fact, the whole town knows. It's an open secret that Max is looking for a new job.

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.

Something that is supposedly clandestine but is actually well known. This term was used as the Italian title of a play (Il pubblico secrete) translated by Carlo Gozzi in 1769 from a Spanish play by Calderón de la Barca, El secreto a voces (literally, “the noisy secret”). In English it came into general use in the nineteenth century for a secret in name only.