We know exactly where it goes!
(And oh boy, it's not exactly a painless area)
Since Lillian has an enigma berry on, I'm happy for her to venture into the horrible corrupted merc guild area, pick up the painful metal disk and use the code on her next turn.

Does someone else want a go with the dagger, in the night? If anyone thinks their type or skills would be good fighting Timpani the Kirlia, we need to interact with them to finish a quest, so we may as well do a friendly battle too, to generate some extra money :D