This article contains plot details from season 1 of Dark. Read at your own risk.
Mads disappeared in October 9, 1986 at the age of 12. He walked back home through the forest after walking Regina Tiedemann to her house because she was afraid of his brother Ulrich and Katharina. Regina was documented as the last person to see him alive in the police investigation.
Ulrich deduces that the body of a young boy found in the forest on the morning of November 5th, 2019, the night after Mikkel Nielsen goes missing, is the young Mads Nielsen who went missing 33 years prior. His body is found with a 1986 pfennig coin on a red string around his neck. This same coin and string is found on the other bodies that Noah sends through time. He also has a walkman that contains a cassette with Nena's song "Irgendwie, Irgendwo, Irgendwann" ("Somehow, some place, some time"). His body is mutilated, with his eyes burned out and inner ear ruptured, which is consistent with Noah's other victims.