Case of the Pilfered Proof is a uncommon Enchantment — Case card from Murders at Karlov Manor (MKM), collector number 9. Current market price: $0.20 (foil $0.16), TCGplayer market prices via Scryfall.
Whenever a Detective you control enters or is turned face up, put a +1/+1 counter on it. To solve — You control three or more Detectives. (If unsolved, solve at the beginning of your end step.) Solved — If one or more tokens would be created under your control, those tokens plus a Clue token are created instead. (It's an artifact with "{2}, Sacrifice this token: Draw a card.")