The problem they'll face is something talked about in any type of situation where natural/accidental death changes a person into a zombie. You are always just one normal death(people die) away from having a zombie inside your 'safe' perimeter/sanctuary.
Now they do have the advantage of it being a prison with lots of lockable gates, which should probably be something they look into implementing during the night and having zones with guards like a normal prison would to watch for a potential person who had died and turned inside.
BTW, did Rick actually leave that woman after she turned? That would seem pretty stupid since he's already had experience finding out what happens when you leave a zombie you could kill. Heck last season we found out what happened to Morgan because he didn't kill his wife when he should have.