No one knows how those marks got on the wall (although dirty pets and hyper children do look pretty guilty), but we know how to get them out. Forget finding the culprit and focus on this easy trick to get rid of the marks:
To clean wallpaper with smears and soil stains, gently brush off all loose dirt and dust. Then go to your bread bin and grab a soft slice or hunk of white or rye bread. You can't use the hardened bread from dinner two nights ago, but you can steal the heel of the loaf that no one ever wants anyway -- all you need is its soft center. Wad up the center of the slice and use it as a natural eraser to gently rub off smears and stuck-on soil.