Season 2: When we first meet Investigator Clausen (Sylvester Groth) in Season 2, we are to believe he is unconnected to the town of Winden and its residents — an outside detective sent to in to solve the case of the six missing persons. But in typical Dark fashion, it's slowly revealed that Clausen is not only connected to Winden, but to The Travellers as well.
Clausen has his eye on Aleksander Tiedemann throughout season 2, wondering why the man took his wife's family name when they married. Tiedemann reveals in an early interview that his surname was actually Köhler, although, Clausen, still suspicious, tracks down records from the Marburg registry office and determines that Aleksander is lying and arrests him for possible identity fraud. It's during further questioning after Aleksander's arrest that we discover Clausen's real motive for taking on the case in Winden and who he really is: His surname used to be Köhler and his brother, Aleksander, has been missing since 1986. Clausen received an anonymous letter promising he'd find answers to the case of his missing brother in Winden.
A photo of him and his brother from the '80s adds another twist: It appears Investigator Clausen's missing brother may be the young Noah who we first meet digging the cave in 1921.