The only time I've ever noticed that is one time that Molli was staring at something in the room and was absolutely terrified. She would hide behind me in my bed and look at one spot in the room, doing a low growl. She did it for several minutes until she finally decided she'd hide under the sheets to sleep. Let me tell you I was scared to DEATH! And I'm a skeptical person about "ghosts", but that night was just creepy.