In Skin Game (chapter 9) Harry tells Ascher that Nicodemus once hired the Churchmice to steal the Shroud of Turin (back in Death Masks). Anna Valmont later appears to confirm this.
By my recollection, though, in Death Masks it was clear that it was
Is this a continuity error, or perhaps a retcon, or did I miss something in Death Masks? Nicodemus did catch up with the thieves suspiciously quickly (Gaston was killed the morning after the theft) which suggests he knew about it in advance, but I don't recall that we ever knew how.