This sounds like an item that a thief wouldn't want to boast about - I could see it making the rounds as each successive thief steals from the last. Probably why the thief was sitting alone.
I like this item and can see a few ways of using it in my own game.
While this may not be useful for direct encounters, I think the presence of these critters would provide excellent flavor for a region in some campaign world.
Of course, it could be that they provide necessary ingredients for some alchemical concoction that the PCs desperately need, or that one has swallowed an important item (possibly while ridding itself of an annoying scholar) which must be retrieved.