What is the mystery that still puzzles Johnny after finding the truth about his parentage?

Johnny was born in Marseilles. After his father died, Vinny returned to America with Johnny and lived a modest life under the name Mrs. Tremain until her death. Johnny also learns that his father gave a false name in America because he was ashamed to have been taken prisoner. Johnny’s real last name is not Tremain but Latour. Johnny still wonders if his father was the French Doctor Latour that had once visited his mother's house.