I used to have a GSD who was very territorial. It always used to worry me that someone would come over our fence and what she would do. We had a small unit in the next street which housed some mentally disturbed men and one day one of them was suddenly in the garden, She sat absolutely still and didn't attempt to touch him in any way, then his carer appeared and she went crazy trying get at him-she most definitely knew that the first young man was vulnerable, I've never forgotten that.