I think dogs have a sixth sense for that sort of thing.

Kia has only gone 'ballistic' once. At my old apartment, my sliding door faced out to the parking lot and the walkway into the building. Some guy was coming up the walk and Kia was barking, snarling, and growling. The guy turned around and walked off. (I didn't recognize him as a tenant)

Kia's never done that since.