Small black dog back at it.

What on earth could a section of wall have about it that so excites my younger black schnauzer? That same section of wall beside the doorway out of the diningroom and into the library. She hadn't been interested in it for several weeks and now she's back at it. Barks furiously at the wall for several minutes at a time. I have it blocked off so she can't dig the wall now but that certainly does nothing to stop the barking.As I have reported before the wall is brick with an overlayer of plaster. It is about 17 inches thick. I have investigated the basement where this wall is built onto a solid granite foundation block. I see no holes, no points for entry. I have left out bait, it remains untouched. The older dog couldn't care less except he sometimes goes to the kitchen to get away from the younger dog's racket. If she keeps it up she's going to get a muzzle. We've about had it. I cannot understand what on earth only she can sense? Seems to me that if it was some creature in the wall the other dog would also be having a fit.