A couple more questions. Are you on septic? Are you in an area with few trees? Are you on a hill? The reason these questions are important, is wind affect. Since your plumbing system (if designed properly) has a vent coming out the roof, as the wind blows across it, a certain amount is diverted down the pipe. If the home is on a septic tank, its like blowing into a coke bottle. The air pressurizes the waste system and bubbles up the traps. Typically it comes up tubs and showers. The wind can put more pressure into the plumbing system than the water in the trap can hold. Since you only have about 2 inches of water depth and water weighs .433 pounds per foot. Which is about 1/2 oz of weight. So if the wind causes a rise in pressure of a pound, it bubbles out the trap mixed with the existing smells in the system. The same thing can happen on windy days in city systems, because the entire system can be pressurized from the wind effect. How this can be prevented, if this is your problem, is by have multiple vents coming out the roof on different gables. Since the gable blocks the wind from hitting two pipes at once, the secondary pipe exausts the air mitigating the effect of the wind.
Another problem can happen if the toilet and shower lines are not designed properly, where the toilet flushing causes a pressurization of the shower line. Since every gallon of water you put into the line causes an increase in pressure because it has to compress the air (in an improperly designed vent system) sometimes it will cause air to bubble out the trap on the shower.