My kitty loves to do this... just sit in one spot and stare at... nothing. She may be listening to something going on in another apartment, or maybe there's a bird outside singing, far enough away that I can't hear it. My personal favorite thing that she does is when she tracks something from one place to another in the room, even swiveling her head to watch it. Naturally, I see nothing, which creeps me out to no end. I call it kitty-vision, and it seems to be her favorite show, which usually culminates with her pouncing on the invisible intruder and munching upon it happily. My cat either locates, catches and eats the tiniest bugs in the world, or she has a great imagination... one or the other. ;)