Not to drag this out forever, but a P trap is what keeps sewer gas from entering your home. Every drain must have one. The trap is built into your toilet(s), some, like the basement shower, may be covered with concrete, others like you sink, are exposed and easily disassembled to retreive rings and clear small clogs. They work by holding water in loop which blocks the gas. If you have a drain that is seldom used, the water could evaporate, so periodically you should run a little water to recharge the trap. If your drain in question is not trapped or if the trap is dry, you would likely smell sewer gas. If it was properly roughed-in, it should be trapped and then the stub capped.