I was out working in my garden when I noticed my neighbor's dog had dug a hole under my fence and was running right at me barking!
Did it scare you or did you make a new friend? It happened to me a few years ago and it turns out the dog just needed a friend. 
Scared the living hell out of me, because he's a pretty big dog! I ran inside and had to phone him up to come and get his dog.
Go over when your neighbor is home and make friends with the dog. Take him a ball or a treat. If it happen again you know your safe.