I doubt she's leaving. A lot of actors work on other projects when their show isn't in production. Michael C. Hall has filmed three movies during Dexter's run. The other Dexter actors pop up all over the place.
Currently Reading: The Martian by Andy Weir
Currently Watching: The Walking Dead, Doctor Who, CSI, Criminal Minds, NCIS, NCIS:NO, Law & Order: SVU, Stalker, American Horror Story, and Bones