#constable clark

You could, of course, suggest that he noticed her when she bailed Watson out of prison following the fight with Dredger, but 1), there probably wasn’t time for formal introductions, and 2) I don’t recall seeing Clarky in that scene. Also, Clarky (along with Lestrade and the rest of the Yarders) don’t attend Watson’s wedding, so he’s not closely acquainted with Mary.

So, when else do we see Clarky? Aside from after Sir Thomas’s death, there’s only one very predominate moment I can think of - the wharf explosion.

He’s the first one on scene, and his line is “Lord Coward has issued a warrant for your arrest - Watson’s alive” meaning that he’s been there long enough to have found and checked up on Watson. And as Holmes runs away, we see Clarky turn and start walking back towards the rubble - where we see Lestrade picking his way over to him. Now, we know at this point that Clarky is married (this is relevant, I swear) and Lestrade isn’t. We also know, that Lestrade doesn’t like getting his hands dirty, and that Clarky does a lot of his running around, as most of his lines start with “Lestrade asks/ Lestrade sent me/ Lestrade says” So, add Lestrade’s lack of wanting to be involved, added to the fact Clarky has more experience dealing with women (remember, he’s married.)

It makes sense that Clarky gets there first, notices Watson, checks he’s alive, realises Holmes is probably nearby (Irene has long since run off by then), warns Holmes that Coward is after him, and by then, Lestrade is just arriving on scene. Lestrade probably at this point notices Watson, and weighing up how they’re going to proceed, says that he’ll get Watson sorted and transported to the hospital, and Clarky can do the messier job - telling Mary.

This explains 1) how Mary even finds out about the explosion in the first place (since Holmes and Irene are both pretty much in hiding) and 2) how Clarky  knows Mary enough to call her by name.