I'm gonna go with "Violet accidentally blew herself up and was caught shortly before Luna" just to make everything simpler on... pretty much everyone. 
Which means that her stuff would still be on the roof, and I'm assuming they took away her guitar case, but would they have taken away her bracers and would they have searched the scene enough to have found her stuff on the roof? If they didn't take her bracers, how thoroughly would they have looked them over? 
Further question: How long should I assume she was unconscious? (Since that'd be the only time they could touch her without getting stuck in her field. I updated her power to make her more vulnerable since I realized I forgot to before.)
Another good question is if they strip-searched the child or if they accidentally knocked against her back to notice the backpack under her clothes. 
Was she conscious or asleep when she was put in the cell, and did being put there wake her up?