Once I caught my dear departed Abyssinian cat, Tut, staring at "nothing" in a corner. I examined the area carefully and found a spider crawling on the wall. Another time he stared at an empty spot on the wall as if something was there. There was. Looking closer, I saw a fly on the wall at that spot.Bogue's good sense is refreshing, compared to the credulity of some pet experts.
Predatory cats are stimulated by movement. Tut may not actually have seen the fly or the spider, but his eye/brain picked up a minuscule movement that caused him to stare at the spot waiting to see if more movement/data would allow him to focus on something.
(Thanks to Stephen Wagner of About.com: Paranormal Phenomena for the tip.)