Well let's look at it from the dog's point of view. He is barking to let you know that he is not sure about something and he needs his pack to back him up with whatever may be happening. So you go out look around can't see anything and walk back inside and your dog is like "Hey! You didn't help me with my problem!" haha so what I have found to work really well is if he does it again let him bark once or twice then go out make a show of looking around then turn to him and say "Thank-you" or "enough" or whatever then take him gently by the collar and move him away and give him a treat. SO you have said to 'it's alright mate nothing much is going on don't worry about it I have it under control' so he learns that he can bark once or twice as a warning but once you say "enough" or "thank-you" it means you have checked the possible threat out and have decided it is safe.
make sense? i tend to ramble sometimes haha
"In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog." - Edward Hoagland