My guess is a minor bug, not really affecting anything.
The pan-stealer was almost certainly Thaler, given the writing style in the notes left behind and the presence of the monacle. I suspect there was once a plan to mention it when you meet Thaler, maybe give him back the monacle, but that the dialogue was never added.
But Main quest's items in the inventory can be dismantled at the blacksmither, and some even sold, when they are useless for the game purposse. And this monacle, as Philippa's agate, and other few items remains in the inventory untouchable.