It feels like you're falling into casual infomodding.
Sherlock is not observant enough to pick up on a title change, but he can guess one single memory with no clue as to who it belongs to is Porthos's, despite his entry indicating nothing about a memory loss and events being able to affect anyone at any point in time, or figure out who Milady is from nothing but her clothes, even though her outfit isn't mentioned in her arrival. She also doesn't look like her book counterpart. Most Miladys have been blonde, as it's one of the key descriptions for her. Of course, he can figure out that if Porthos, who is usually a big fat white man, can be a mixed race fit one, Milady could look different from expected, but this isn't said. He goes by simply an assumed outfit that's apparently so distinct he'd have no doubt about her identity.
I understand Sherlock is Sherlock Holmes and making huge guesses based on very little clues is part of what he does. But there's a thin line between being an observant genius and simply info-modding and making assumptions based on pretty much nothing.
I would suggest explaining his train of thought in better detail for things like these, because otherwise it only feels like you're using OOC knowledge for these things. A glance in her direction tells him she's probably familiar with the Musketeers. A phrase like that sound great, except it should be followed up with an explanation.