Continuity mistake: After Prince Philip's body is placed on the bier, Mulan wraps the wraith's medallion in a bag and gives it to Aurora. In the wide shot, the strings on the bag are hanging over the left side of Mulan's hand. In the close-up, they are hanging down the right side. (00:33:25)
Continuity mistake: In the scene where Mulan and Aurora are alone in the forest and Mulan is trying to make a fire, she alternates between wearing her long ponytail over one shoulder, and letting it hang down her back. (00:24:45)
Answer: He knows who she is because he knows that the fairy tales are true. He was also shown to sneak out to go see Emma or someone else. Sometimes he succeeded, sometimes he didn't. This is how he knew to look for her despite his strict mother.