Why does Mary not recognize Sherlock?

 (Sherlock Meta by Ivy Blossom)

It seems odd to me that Mary does not recognize Sherlock when he shows up at the restaurant in The Empty Hearse. 

If she is secretly employed by Moriarty, or is Moriarty, or is in any way related to Moriarty and his various and nefarious enterprises, either as an assassin, a colleague, a family member, or whatever, surely she would recognize Sherlock right off. If her purpose in lying to John and staying close to him is in any way related to Moriarty’s plans for Sherlock, she definitely should have.

If she is none of these things, and is merely a secret contract assassin with no ties nor interest in Sherlock Holmes whatsoever, it’s still odd hat she didn’t recognize him, given a) Sherlock’s notoriety, combined with b) her boyfriend’s past and his current status as Sherlock’s mourning best friend. She’s surely seen pictures of him. How did she not recognize him?

If she did recognize him for either of these very reasonable reasons, but chose to pretend she did not recognize him when John did in the restaurant in The Empty Hearse, what purpose could it possibly serve?