Earlier in the story, Door and the Marquis visit Door's father's private study. The main focus of this scene is finding Portico's journal, which tells them to find Islington. Briefly mentioned though, is that the Marquis finds the token on Portico's desk and pockets it:
Meanwhile, the Marquis examined the objects on Portico's desk. An inkwell; a chess-piece and bone die, a gold pocket-watch, several quill-feathers, and...
It was a small statue of a boar, or a crouching bear, or perhaps a bull. It was hard to tell. [...] He picked it up casually, turned it over, curled his fingers around it.
The Marquis slipped the carving discreetly into an inside pocket.
Door doesn't notice. Interestingly, the Marquis does this even though he's apparently not sure what it is. Good instincts, I guess.