Noun
something that is supposed to be secret but is generally known
Source: WordNetReason is like an open secret that can become known to anyone at any time; it is the quiet space into which everyone can enter through his own thought. Karl Jaspers
Art like life is an open secret. Lawrence Durrell
It is a blatant, open secret that this attack on the Twin Towers was done by George Bush himself. Zakir Naik
It is a fairly open secret that almost all systems can be hacked, somehow. It is a less spoken of secret that such hacking has actually gone quite mainstream. Dan Kaminsky
their love affair was an open secret Source: Internet
Among those familiar with his poetry, his identity became an open secret, but Scott persisted in maintaining the façade, perhaps because he thought his old-fashioned father would disapprove of his engaging in such a trivial pursuit as novel writing. Source: Internet