Yea...dogs are a great early warning system...my female yellow Lab is not a barker, until someone starts up the walkway to our house. Then she barks...and if we are not in sight...she will come upstairs, to the basement, or anywhere we are located, look at us, bark, and throw her head as if to say " yo, com'on " and then walk towards the source of her worry, looking over her shoulder to make sure we are following...
Very smart...otherwise, very docile. People visiting often ask if she ever moves...as she lays very still...taking in the conversations, until, another guest starts up the path...then they see her spring into action.
My wife likes to walk her when she comes home from work...on a leash of course. When a strange dog without any owner in sight started to approach my wife, the Lab went ballistic...even to the point where my wife said she bared her fangs, and the hair raised on the back of her neck, as she struggled to get to the threat with my wife trying to hold her back.