I have a strange smell (somewhat like decomp but its not) that has presented its self every August since 2009 and stays for 14 days. It comes on strong the second day and stays the same smell level for 12-13 days and goes away until the next year. I have had the plumbing inspected twice with a camera and 2 pest control companies that say its not rodents. I have bored holes in walls and used video scope to inspect inside walls looking for any signs of pests and have found nothing. The house is 2 story built in 2007 on concrete slab with no drainage issues around the house.
Could there be a hidden floor drain in the area? It wouldn't explain why the timing is so regular, but it might be connected to something that does. I've dealt with a similar come-and-go odor problem in a commercial buidling that ended up being a 4 inch open floor drain that was just carpeted over when the building was turned into offices. That drain line connected into a drainage system that was tied to a nearby building that housed a chicken processing operation. You can imagine what sorts of things were washed down their pipes. It typically only smelled during the dry season, I suppose because some elbow trap between us and the odor dried out.