"Agatha Heterodyne and the Electric Coffin" is the title of a short story originally published as a bonus story in the print albums of the "Girl Genius" comic. The web page linking to the story on the Airship Entertainment website says it is: "An authentic and true (we swear!) tale of Agatha's later adventures."
In the eight-page story, Agatha, aided by Krosp, is doing the "Lady Adventurer" thing, tracking down problems and trying to fix them. She has come across the activities of a gang of "rogue constructs" — Zon, Garon, Luilla, and Torsti Lechwa, whose creators have died or abandoned them. They are surviving by working as confidence artists, and the "Electric Coffin" of the title is an integral part of their audacious plan to avoid detection and pursuit.
The end of the story portends some interesting developments for Agatha's future career.