Possible Clues:

• Something many people know although it has not been explicitly divulged
• Strange presence in part of Bible that everybody knows
• Common knowledge
• What everyone knows that they shouldn't know
• Someting poorly hidden
• Traditional oxymoron of MI6, say
• One's generally known to pose with centre fold
• Begins with European island - almost everyone knows it
• No point keeping this to yourself as it's off the latch and underground
• Respect one formulating a 'known unknown'