Paige says scrying won't work because Billie's parents aren't magical. However, a few episodes before Paige herself scrys for an innocent, whom isn't magical.
The symbol in Billie's sister's diary at the end of this episode is the Heartagram, the sign for the Finnish "love-metal" band HIM. Specifically, the heartagram shown in the diary is the variation that the band's frontman, Ville Valo, has tattooed on his stomach.
Piper posing as Dalvos is punishing the guilty, yet in season 2, morality bites, Phoebe was essentially burnt at the stake in future for doing same thing