From Halopedia, the Halo wiki
Super Sleuth is an Achievement in Halo 3: ODST. It is unlocked when the player finds the final clue to unravel the mystery, either alone or with another ODST. It is worth 10 Gamerscore. It will unlock upon beginning the Kikowani Station mission, having started a complete run through in the correct order.
There is a Marathon symbol in the background, behind the actual symbol, in the gold circle.