Mary knows what it took for Sherlock to carry out his plan to live in hiding for two years. She’s gone so much further than Sherlock – erased her entire past (or tried to), assumed a completely false identity. She knows Sherlock would’ve needed a confidante, because she needed one too. So who was hers?
Janine.
Let’s walk through season 3 and take a closer look at Janine. We meet Mary in The Empty Hearse, but we actually get a peek at Janine before that. Did you notice her? She was in Many Happy Returns.
Granted, that’s as close as we get. But it’s not just the hair, the face shape, the body type. Her clothes are a giveaway, too. Feminine, pink, sweater set.