When you go to jail you are relieved of your street clothes and personal items.
Charmcaster says she's lucky that Gwen has a magical aura about her, otherwise she wouldn't be able to cast spells while in Gwen's body. Her grand plan is to possess Ben's body so that her spells will be alien-supercharged. She forgets that Ben doesn't have a magic aura about him. That would make Charmcaster incapable of casting spells while in Ben's body.