Spooked in backyard

Tonight we were out back when Zach ran to back door, with nose in the air. I heard nothing, and thought he smelled something. When we came inside he ran from room to room barking and crying. It took at least 10 mins for him to settle down. Even after he settled down, he stuck to me like glue. Two hours later he did not want to go off porch to go potty unless both of us went out with him. Again, I heard and saw nothing. I know dogs ears and nose are better than humans, but I have no clue what spooked him.

"Happiness is being loved by a Westie puppy"
zachary_fan_club@yahoo.com