My dog barks at ghosts??

Hi all,
My 2 year old Aussie Shepherd cross named Belle often gives a strange little bark when she's lying down on the floor, resting. She's always awake, so it's not that she's dreaming, and there's nothing moving in her line of sight. She always gets up right away and changes position though. I wonder what that's about? Anybody else have this experience?
thanks!

Kathy
Mom to Belle (2 year old Aussie Shepherd cross), Yukon (13 year old black lab) and to Jackson the cat.

she may hear something mice or something like that i know my cats will stare at awall when the mice or squirells get in

my terrier/collie/somethingorother X randomly does weird quiet-ish barks in the evening when she's awake and lying down. I figure she's heard a dog barking somewhere, and that it's not audible to me.