Headscratchers / Casper Meets Wendy

• How do the witches know about Desmond's magic tracker as it was only revealed to his two henchmen: Jules and Vincent at that point?
  • They don't, but they're probably aware that as they put it, "big magic sends out big static" and methods to track them through it exist.
• Why are ghosts and witches each other's enemies? While the ghosts in some ways have a reason to hate witches with the line coming from Stretch, "Witches are 'fleshies' with power!" as in they can actually fight back with ghosts with their magic, there is no information from the witches' point of view, as to why they hate ghosts.