Some dogs are just runners ! I had Brittany Spaniels that always took off and we had to catch them. As far as the other thing you're asking about, he may have eaten something while he was out and it may be bothering him. Best advice I can give you is to call your Vet if this continues. I don't think it's any kind of "phase" but maybe he had a traumatic experience when he was gone. He might be thinking of something that happened out there. Call your Vet to be sure he's OK. Better safe than sorry!
My dog keeps running off, he was fine all day until tonight. He just went a-wall, he doesn't want to look at me and is acting as if he is seeing something. Is he going through like a phase or something? What's wrong with him?
I agree with Rooster...Some dogs will just run. My Siberia Husky had to be leashed outside as if she got loose, it was forever getting her back! However, if you feel your dog isn't acting right, I would definitely take him to the vet to be checked out.