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Wiktionary
open secret

n. 1 A fact that is widely known, but not acknowledged openly. 2 (context dated English) Information that is not widely known, despite being freely available.

WordNet
open secret

n. something that is supposed to be secret but is generally known; "their love affair was an open secret"

Wikipedia
Open Secret

Open Secret is a 1948 American film noir directed by John Reinhardt, starring John Ireland and Jane Randolph.

Usage examples of "open secret".

Sharon figured it was probably pretty much an open secret by now that messages came and went through the secret room, but the local staff seemed to be pretty good about at least keeping up the fiction that they didn't know what went on in there.

It was an open secret that diplomats didn't trouble themselves with customs duties for such things as perfumes for their wives (or significant others) and/or booze for themselves, but to Ryan's Catholic measure of personal conduct, these were venial sins, not mortal.

It was an open secret now, within the diplomatic community, that Don Fernando had privately written to Urban VIII asking whether a petition for laicization would be looked upon with favor.