He asks that in an interesting way that piques her curiosity, and she tucks that observation away along with all the other little things she's noticed about him.

"There was a mirror," she answers. "I couldn't see myself, but saw Sherlock. Or...at least I thought it was Sherlock. It looked like he was asking for help, but when I touched the mirror he grabbed me and tried to pull me in. That said...I wasn't the only one who saw the spirit in the mirror. Although I might have been the only one to see the particular form it took."