Mysterious Smudges

I was cleaning and noticed some strange black smudges on either side of my hallway walls. I went to get my cleaner and my paper towels trying to guess at what these may be. As I was cleaning I noticed that they went all the way to the ceiling. I was still racking my brain, trying to think what may have caused these. Then I remembered. A couple weeks back, Grace hollered at me from the hall. I looked and didn't see her. She hollered again and this time I looked up. She was on the ceiling. Mystery solved. Next time, she's cleaning the walls.