My girl does that often at night. I can't hear anything, but she'll perk up all of sudden...sometimes growl, sometimes bark, sometimes bolt off in a certain direction. I've always assumed that she hears something I can't hear. I've just learned to live with it. Once she warned us that an elderly neighbor was in trouble. The husband had a seizure & the wife was lurking around the street trying to find someone with lights on that could help her. She's our little guard doggie
