The drains have their own systems to prevent odors. In all cases there is a siphon, which is nothing more than a U-shaped pipe, which retains a certain amount of water inside, preventing odors from reaching the pipe.
I could be that the water that is inside the trap, to evaporate. That is very common when not used for a long time, like a summer house. In this case the solution is very simple. You just have to leave the tap open for a long time and fills up quickly.
Also possible that small food debris to accumulate in the trap. This is for having the bad habit of allowing to happen. These remains are decomposed causing unpleasant odors.
A simple solution is to mix 100 grams of sodium bicarbonate in 500 cc of warm water and 250 cc of vinegar. Pour down the drain and leave for several minutes.Then we just have to pour at least a quart of boiling water, and problem solved.
