Sydney's last name Sage is both an herb, and another name for a witch.
There is a book that appears to be about magic in the Alchemists' library in The Indigo Spell. This foreshadows Sheridan's knowledge of magic users.
The Reveal in The Ruby Circle that Sydney's father was one of the Alchemists selling Moroi blood to the Warriors makes Keith being the person behind the tattoos at Amberwood in Bloodlines make even more sense.
When Sydney confesses to being a witch, warden Grace Sheridan reveals she is not the first human magic user the Alchemists have captured. She in fact knows from past experience that magic users are more resilient to pain, and she starts subjecting Sydney to more painful torture. The question is what happened to the other magic users, knowing that the Alchemists are an Antimagical Faction. Were they ever released? Or even scarier what kind of experiments were done on these witches?
When you realize that Grace Sheridan is the textbook definition of a sociopath, someone who is outwardly charming, but underneath enjoys causing pain. And either the Alchemists are unaware of this, or it's the reason why they put her in charge of the Re-education center, because they feel that she is useful there, torturing disobedient Alchemists into compliance.
