When I worked as an apprentice electrician I have also seen old houses that were remodeled in which the plumbing lines (sewer) were leaking in the basement for years, mostly owned by elderly or disabled people, or landlords that just rented them out. My point is that the water would absorb into the concrete and soil, and everytime it was damp it would smell pretty nasty down there, I would tell the person they had a leak and there response usally was its been leaking for many years and I have no money to fix it. So if you find no vent issues I would check under that carpet and see if the smell is coming up from the ground.Good Luck!