Coincidence
"A coincidence! ... The odds are enormous against its being coincidence. No figures could express them." ~ The Adventure of the Second Stain.
It is only his years of acting training that prevents Holmes from showing any reaction to the name "Mary Morstan."
He remembers well the previous holder of the name, even if Watson, it seems, does not. And so he resolves to keep a close eye on Miss Morstan, and a closer one on Watson.
A good assistant is hard to come by, a good friend even more so.
And Holmes is not a believer in "coincidence".