I didn't read the novel yet but I have a speculation that the hunter association leader at the early chapter who tried to bid for the portrait is one of his students.
It's describe that person doesn't have any particular hobby of collecting artifacts yet tried to win the bidding for the portrait, My theory is that person is his former student who recognized him and knows he went missing in the fire.

Since Jio used to be a teacher before he died... And also since he died saving his students(?) , does his old student recognised him ? I mean if my teacher died saving me I would remember them for the rest of my love lmao