Whilst they're in the asylum, searching for Frank, Kai once takes a short look into a room with a patient in it. That patient is Achim, a former undercover cop, who were after Kai and Stefan in the first movie "Lammbock" and which got knocked out by them with psychoactive mushrooms and a pesticide "Gehirnzellenmassaker" (german for "braincellmassacre").
Moritz Bleibtreu (Kai) wrote the rap song in the credits.
Comes as "Lommbock" to German cinemas in March 2017