There are traps in every drain because any connection that leads to the drain system is also a possible outlet for sewer gas.
When someone reports a strange odor in any room where there is a drain the first thing I would check is to make sure that the trap is not dry. If a drain trap is dry the sewer gas can escape and can be the cause of the smell. This is usually a quick fix that can be remedied by running water down the drain and filling the trap back up with water.
The location of the drain trap is sometimes obvious such as in the toilet where you can always see the standing water in the bowl. If you look to the backside of the toilet you can see the shape of the drain through which the water exits has the necessary curve.
Other plumbing fixtures have the drain trap in an out of the way location such as the kitchen or bathroom sink where the trap is usually hidden in a cabinet under the sink. When you look at the sink you cannot see the standing water but if you follow the drain lines you can see the required U or S shape where the water stands to trap the sewer gas. Sink traps have the added benefit that they trap small objects dropped in to the drain and they are fairly easy to remove. So not to worry if you drop a ring of something valuable down the drain because it is likely safe in the trap.
Large plumbing fixtures such as showers, tubs, and washing machine drains also have drain traps but they are not as easy to see because they are under floor level or behind walls. Tubs and showers have traps that are harder to get to and either requires crawling under the house or cutting a hole behind the tub or shower and dig out the area where to trap is. Washing machine drain traps are mostly in the wall and to access it you have to usually cut the wall as well.
All drains should be used at least once every couple of weeks to keep water in the traps. This includes showers, toilets, tubs, showers, bathroom sinks , kitchen sinks, wash machine drains , and more. It will not hurt anything to let the trap dry out but you may get some smells that are unwanted creeping into your house.