During the Troubled by Vermin storyline, you can interrogate the rattus fabers to get them to lead you to one of their stashes. This will get you a randomly selected treasure. The glim, beeswax, rostygold, and suits of clothing are all things you'd expect to find in stashes like that, and even the Infernal Contract doesn't raise too many eyebrows...but how the hell did they get hold of human souls?!
Souls are a tradable commodity. The rats aren't necessarily the ones who took them from their owners.