How could he disguise himself? I only have some idea about late-nineteenth gentlemen's clothes. I'm not sure whether common people would wear any kind of cowled capes, broad-brimmed hats or other things that'd help him go unnoticed in dim light, in places where no-one looks too closely at faces.
So, how can the Phantom of the Opera conduct an investigation among Parisian lowlifes and not get busted for high-profile murder, vandalism and gatecrashing a certain masquerade?