Drama for Your Momma

Well, I can't say that Cerb and Mara would keep things a closely guarded secret, so it's possible they'd freely mention taking someone captive to someone they were talking to. Possibly a friend/relative who would relay what they heard.
Or, since Mara's kind of gone through some changes after having kids and all she could feel guilty and try to find someone to tell about Cerb capturing her, since she'd be unlikely to actually try to free anyone from her brother without backup. If you wanted to get more in depth perhaps Felicien could find out what they did and be pissed and try to find where the girl came from.