smell in basement

We recently moved into a bi-level house. Shortly after, we noticed a very sour smell coming from the closet below our basement stairs. At first we thought it was sewer gas coming from the cap. We have since had a camera through the lines, as well as our drains augered and still have the smell. The smell has however, seemed to have moved into the hallway at the bottom of the stairs. Since we did not have any luck with the plumbing system we thought it could be from our vents, possibly a dead animal. We have cleaned out the vents and looked in the ceiling for any dead animals, but still have not been able to locate the cause of the odor. If anyone has any suggestions, I am will to try about anything.