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.