I would take that to mean someone you feel you could trust with your life or someone you believe is really close to you, could actually be stabbing you in the back, without you knowing it i.e. passing on confidential conversations you are having with your "Spy" to someone you don't like. Someone you work with passing on information (Like the example above) to your Superiors, so you get overlooked for promotions or you get the really bad jobs to do. You keep their worst secrets to yourself but they rush out and tell everyone all of yours. That sort of thing.