So, WoJ has it that Mab is "clearing her debts", balancing her scales and paying back favors to get free from obligations.
What happens if there is a tiny favor that she owes to a mortal who then dies before repayment?
How could this be? I am positing a mortal human who: is an only child, whose parents are deceased, whose grandparents are deceased, who has no aunts/uncles (never existed or deceased without issue), who has died without essentially any other living family.
His material estate would (usually) be escheated to whichever unit of government of his residence/locality/citizenship handles such things, but what about a favor owed by Mab? If he had had descendents, I think the favor would be passed down as part of the bloodline "inheritance", as it were, the same way that the favor owed by Thomas to Cat Sith is still owed to whichever entity becomes the new Cat Sith, but where does the the favor owed to a deceased childless mortal go? Mab is still bound to pay it back somehow!