Most likely a previous Warden, Jim won’t say who the last two Wardens were and I think that the last was Fortier as a placeholder, an absentee to prevent Kemmler from regaining the Wardenship which he had previously held. He had no interest in Demonreach, which made him perfect after Kemmler. His murder was to free the wardenship for a Black Council Member for access to the ley lines except some idiot blundered into it instead. The only definite info we have on Wardens are Merlin, Kemmler and Harry. Harry is the only local.
Humans can’t be held permanently according to WOJ but not permanently Could still mean centuries and Thomas falls within that category according to Demonreach so presumably would a Wizard, so they could have been in there centuries, a blink of an eye for Demonreach which thinks in geological time as a genius loci. That’s one reason I think it’s Bob’s dad, hiding from Lea.