1. A dried-out water trap under a floor drain or washer tub. Water in any trap under unused drains will eventually evaporate, allowing sewer gas to come up through the drain into the room. Fix this by dumping a pitcherful of water into the drain to restore the trap water. Create a floating seal of oil that will keep the water from evaporating so fast next time by pouring a couple of tablespoons of cooking oil in top of the water.
2. A missing cleanout plug inside the floor drain. Remove the grate that covers the opening and make sure there’s a plug inside the drain bowl. If the plug is missing, there’s a direct path for sewer gas to bypass the water trap. Sometimes, the plugs are removed to clean sewer lines and not replaced. Buy a replacement plug at a hardware store.
3. It’s less likely, but the water in the toilet trap also could have evaporated. Weeks of disuse could cause this. Simple flushing will restore that water.