I'm pretty sure the girl who has been absent is the one actually in love with the main character. Got super embarrassed and abandoned going to school altogether. Doesn't think the MC remembers her.
Miki then goes to that girl's house and stays over sometimes, when she does she's hoping that our MC will recognize that shampoo and therefore be thinking about the absentee.
Done. Now if there are other people with supernatural abilities that would be an interesting mix-up