All drains and fixtures in a dwelling are supposed to be hooked to a vent pipe that extends up through the roof to expel back pressure in your plumbing system. Drains are protected from gas escaping into a dwelling by a "gooseneck" or water trap in the drain. This is true for floor drains, sinks, and toilets. If this water in the trap evaporates because of extended non-use, the gas can escape into the dwelling. A simple solution is to add water to that drain and any drain which is not used that often. If this does not solve the problem, it would be a good idea to have your venting system inspected.