Possible Clues:

• It's common knowledge, Frank has a skeleton in the closet
• Nevada's Area 51, notably
• Something that's common knowledge but should be confidential?
• Ill-kept confidence
• Poorly kept confidence
• No point keeping this to yourself as it's off the latch and underground
• Centre pose (anag.)
• Everyone knows it, but they shouldn't
• *Officially restricted yet widely known information
• A known unknown?