Schmidt's cell door.
Schmidt was a murderer held in solitary confinement in Shoalsgate Station. Lt. Hagen ordered Schmidt to be left there to rot as he wanted him to suffer as much as his victims did.[1]
Schmidt's cell cannot be accessed but his ramblings can be heard from outside. He is quite clearly psychotic and speaks in similar manner as the Hammerites or Mechanists. Judging from his ramblings, it seems he murdered members of one of those two orders.
Interestingly, Schmidt seems aware of Garrett even though he cannot see him through his cell door. He claims that he has heard "them" speak of him, and that his "relationship with the shadows will never know betrayal".[2]