But then what does “surveillance status. Grade Three Active” mean anyway?
It means he’s SUPER SUPER SERIOUS about having Sherlock and John spied on! Grade Three! That’s at least two higher than Grade One! CAN’T YOU FEEL THE DRAMA?
Hahaha, I’m pretty sure they just throw in that ridiculous jargon to sound cool. Then again, from what I understand, that also seems to be why the military and intelligence community do it, so verisimilitude, I guess? (Or maybe that’s just American military/intelligence.)
So the short answer to your question is probably Plot holes. Giant goose-honking fly-south-for-the-winter plot holes. You can either accept the logic you’re given, or laugh and say “No, but really now” and then play jigsaw puzzles with the text to come up with a reason you like as to why it might actually make sense. Or you can hope that maybe that story arc isn’t done yet, and next season we’ll learn more that will make it make sense.
But you know what? Now that I think about it, why are we assuming Mycroft DIDN’T know?
(Spoilers down thar.)
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Equally possible is that he DID know, and he never said anything because he assumed that Sherlock and John already knew and just didn’t care. After all, Mycroft himself sure as hell has no reason to be fussed about her. She’s his normal work week. He has business lunches with people like her.
And he would assume that OBVIOUSLY Sherlock noticed when he first met her, right? (After all, we’re still asking ourselves how he didn’t.) And I doubt he’d see any reason why John would mind, considering how happy he is with people like Sherlock. In fact, I doubt Mycroft physically hangs around John much, so he may simply figure that Mary came clean to John early on. After all, when it comes to John Watson, being a morally dubious professional wanker is a point in the Plus column.
And Sherlock and John DON’T care all that much, do they, once they figure it out? Sherlock’s pissed that she tricked John (and maybe that she shot Sherlock), but he doesn’t give a damn what she does for a living. And John has a few months of angst, but is that because Mary was a professional killer or because she lied to him so thoroughly?
(Now I wonder what it would’ve looked like if Mary HAD been honest to John before they got engaged—at least that she used to be a spy, if not the bit about being a contract killer. Would John have minded as much? I suspect he would’ve banged his head off the table a few times, sighed heavily and then gone right on with things.)
In fact, now that I think about it, this may be one of two possible explanations* that makes sense of why Mycroft didn’t blitz her off the face of the planet for nearly killing his brother. If he assumed they already knew, then that whole scenario looks less like a betrayal and more like one of Sherlock’s plans gone horribly wrong. And by the time he realized that wasn’t the case, Sherlock had already sorted it.
*The other possible reason that might make sense is if we discover next season that Mary is a double-double-crosser and was actually working for Moriarty or something, and Sherlock and Mycroft have been running this plan the whole time.