Possible Clues:

• Detective does a twirl in front of Number 10 - presumably, it's revealing who he really is?
• Proof of membership
• Bouncers might request this if you're going to music halls perhaps to see bands at The Centre from Dido to Queen
• What a student may swipe
• Driver's license, e.g., in brief
• Proof of who one is
• Item in a student's wallet
• Document with photograph, age etc identifying a person
• Access requirement, at times
• It might open doors