Usually odor inside a house is caused from a dry drain. Each drain in your house has a trap associated with it. The trap is the U-shaped pipe you see under your kitchen sink. The purpose of this pipe is to keep the gases from your septic system from being able to come back inside the house. The trap works because water sits in that U-shaped section of pipe, and therefore the gases can’t get back up through the drain.
However, if the water in that trap should ever dry up, then the gases are able to escape. The water evaporates out of these traps if they aren’t used for a period of time. Usually it’s an unused guest bathroom, or basement shower stall, or some fixture that just isn’t used very regularly.
The solution, add water to the drain!
This odor emanates from the vent stack on the roof. Each drain in your house has a vent stack associated with it which is the small pipe(s) you see sticking out of your roof. These pipes are designed to vent the gases from the septic system out through the roof. They also help with the drainage of wastewater through the plumbing system, keeping air-locks from forming.
Depending on where the vent pipe comes out of the roof, the height of the pipe, the height and shape of the roof, and the direction and speed of wind, the gases could wind up back down on your patio due to a downdraft.
Odors could also be coming from the foundation vents for the crawl space if there is septic seepage from the sewer pipes before leaving the house to the septic tank.