Emma was walking down Baker Street, alone. Mycroft tried to make her take her body guard with her, but she insisted on going alone. If Ian, Moriarty, whoever he is, could hack into Mycroft's computer, he could easily take down two body guards. And she didn't want to hurt anyone else anymore.
She didn't take a cab, since Mycroft persuaded her that the cabbie might kidnap her. She thought it was highly unlikely, but as Mycroft asked her to not do so, she didn't.
She looked around at the people in the street. It felt awfully empty to her.
She noticed a man on the opposite side of the road. He was wearing all black clothes, but not in an unusual way. What disturbed her the most was that we was staring at her. He wasn't simply looking around - his eyes were fixed on her.
She slowed down, her eyes on the strange man.
He moved a bit, and she saw a small black object pulled out of his coat. Just as she realized what it is, there was a large noise and the man ran away.
Less than a second later, she felt a sharp pain in the left side of her body. She put her hand on it, and then removed it and looked at her hand.
It was red with blood.
Emma, panicked, put both of her hands on the wound, and walked as quickly as she could towards her destination.