I'm reading Luc Sante's
The Other Paris. I find that there are surprisingly large gaps in the historical Record of Paris caused by Paris' often massive urban renewal projects. There would be multiple groups of historians and cartographers in just about any echo of Paris.
Given that in the default Infinite Worlds campaign Homeline's France seeks influence in every alternative France, This means that the Cabal, Centrum, Reich-5, Swagmen, and others, can be reasonably assured that if Homeline knows of a parallel there will be some signs of this in Paris.
More fun, the underworld of Paris is aggressive, clever, possessed of a wicked sense of humor, and aware of those places the police ignore. Thus the locals will be in on the act at the worst times.