WordReference Random House Learner's Dictionary of American English © 2014
ˈo•pen ˈse•cret, n. [countable]something that is supposed to be a secret but is actually known by others.

Collins Concise English Dictionary © HarperCollins Publishers::

open secret n
  1. something that is supposed to be secret but is widely known

'open secret' also found in these entries:

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