The Guardians of T`Vyl drop the Platinum, I believe it was the Servant of T`Vyl (male half-elves!) who dropped the Gold and I got a Bronze from the Dark Followers (DE females) all over the first level. I am guessing that there is at least a Silver Sigil also, probably on the 2nd floor, but I didn't explore there. I would assume that they are a part of a quest, possibly not yet implemented.
Bair!!! Heh..good to see someone I know posting on here :)
Looks like a quest to me too, but probably going to be a Neriak quest meaning most of the Paladins and Clerics that liked to go in Befallen will have no use for these.