My Annie did this awhile back. I have tall cupboards and one night she looked up at them and never took her eyes off of something and walked along the entire room like that. I never did see what she saw.
Leena has several times looked into our bedroom and the kitchen, growling at something we couldn't see. She'll do this for a few minutes at a time, and I'm sure it's not a "noise," because she turns her head to the side at odd noises.