urine smell in basement stairwell

There is a urine smell two steps down into the basement. This seems to be where the odor is concentrated. It gets worse as the house heats up during the day. There have not been any pets in the house for over a year. I've checked the attic for water or dead animals and have checked the basement. The odor does not consume the basement just the top two to three stairs. It's not the stairs themselves. It seems to be in the air or possibly in the wall. Any ideas?