Dog continues to bark at the wall.

My younger dog, a four year old black, female, schnauzer barks at an area a couple foot square on the western library wall just a few feet from the library door that leads to the diningroom. I wrote about this before but the barking continues. I was working upstairs most of the morning yesterday and I could hear her barking at the wall almost the entire time. Luckily, this old  house is blame near soundproof or that continual barking would really get on your nerves. Same place, every day. Sometimes she frantically digs at the wall while barking at it. Believe me it is getting old. I've checked the area thoroughly and found no evidence of mice or anything else (The walls here are solid brick and 17 inches thick).Besides I have an older grey and white male schnauzer who pays no attention to the wall. I think her barking is getting on his nerves too as he tends to go into the kitchen and as far from her as he can when she is barking at the wall. If she can hear something in that wall, you would think he could too. But either he doesn't hear anything strange or he choses to ignore it. These barking spells can go on for 15 or 20 minutes at a time several times a day. So is the house weird or the dog?