Revealed as a used Suppurating Eye scroll — a planted spy-camera the enemy used to eavesdrop on the party.
The Mysterious Letter, looted from GumGum's inventory, is finally understood: Mordecai identifies it as a used Suppurating-Eye necroscript scroll, deliberately planted (likely on a dead prostitute) so investigators would pick it up. Its blood is the caster's own — meaning that whenever the letter is out of Carl's inventory, the enemy can see and hear everything around it. That explains how the murderer overheard Carl swear vengeance and dropped a mutilated corpse minutes later.
Princess-Donut flips it into a weapon: they place it on the roof and perform a fake conversation about breaching the magistrate's office, then feed it to a clockwork Mongo that leaps to the wrong building — luring the watcher's forces away before Carl's dynamite blast.
A blood-covered necroscript scroll; the blood is the caster's, not the corpse's it was found on.
An enemy surveillance tool the party turns into bait for their misdirection play.