Sewer gas odors sometimes enter a home because the water in a seldom-used trap has evaporated or something is blocking the vent pipe. Preventing the odor is sometimes as simple as pouring water down the drain but might also involve going on the roof to clean a vent.
Plumbing systems use traps to prevent gases from entering the home. These traps are U-shaped pipes that hold a small amount of water. Adding more water to the drain forces the water down the pipe while refilling it with fresh water. Without adding water regularly to replenish that in the trap, it evaporates, allowing gas and odor to enter the home. Floor drains are common culprits, but a drain in a remote bathroom also has the potential to evaporate and allow the smell in the home.
Vents on the roof of the home allow sewer gas to escape and provide fresh air so water flows freely down the drain. Because the vents are open on top of the home, they sometimes become filled with debris that prevent the gases from escaping. Over time, the pressure builds so it is sufficient to bubble back through the trap, causing a sewer gas smell. Cleaning the trap with a plumber's snake usually eliminates the problem.