Possible Clues:

• Something that's common knowledge but should be confidential?
• Nevada's Area 51, notably
• Ill-kept confidence
• It's common knowledge, Frank has a skeleton in the closet
• Unspoken shared knowledge
• A known unknown?
• Centre pose (anag.)
• Everyone knows it, but they shouldn't
• *Officially restricted yet widely known information
• Private matter actually known to many