The key Tipid says he lost, mourned in the same breath as thinking he was finally going home.
After Rosetta explains that his brain repairs have been ruined and that he cannot safely leave the dungeon, Tipid looks up from his hands.
He says he thought he was done, thought he was going home. Then: I lost my key. Rosetta answers by calling him comrade, sitting down beside him, and offering to tell him a story about the bravest Crest she has ever met.
The lost token standing for Tipid's forfeited way out of the dungeon.