Hersheimmer had left for Constantinople at a moment's notice if he fancied that a clue to his cousin's disappearance was to be found there.
Constantinople is where Hersheimmer's cousin disappeared. 
mnli hypothesis: Constantinople is where Hersheimmer's cousin disappeared.  premise: Hersheimmer had left for Constantinople at a moment's notice if he fancied that a clue to his cousin's disappearance was to be found there.