Based on what you wrote, I don't see this as bugging. The dog is locking in on something, dogs that are bugging do not do this, they avoid looking. Also, I would ask, how do you know what your dog is looking at? Just looking at the back or top of their head won't tell you what their eyes are focused on, especially if what you think they locked on is fairly close to the pile. Have you done any pop-up marks to get a read on the dogs head/eye set to better understand this?