i'm no expert, and i won't claim to be, but my dog will run scared from one particular friend of my step-dad and we never knew why. It was so weird, though later we found out that the dog had gotten overly friendly with one of his kids and he'd told him off. Ever since, he never acts right around him.
maybe something similar has happened to your dog? Some kids down the street or something like that? It could be that he got scared cuz he got out and didn't know how he got in; remember dogs are like children in many ways and can be very needy.
i'd suggest to give him lots of love and see if he gets better; if not, you could allways take him to the vet or ask your neigbours if they saw anything??