"When did you see her, Mary?" she asked presently. "You seemed to know her when we stopped at that bed."

"I saw her once or twice, in the Regent's Park, waiting about for someone. One day a man came—I only saw his back. Then a long time after I saw her again, one day at the end of July. She waited a long time that day, but he didn't come." (Ch. 20)

This is a location mentioned where someone saw Mary. She was trying to meet a man, but a man who Allison had never met or heard Mary talk about. Allison says, "Lots of girls would have made a scandal, out of revenge." (ch. 20) which hints that maybe Mary potentially has a love affair going on with this man.