How often is the bathroom used?
I have a small powder room that rarely gets used. Because of this the water in the elbow under the sink dries up... causing the sewer smell up through the sink on occasion.
I just run water in the sink and check the water level in the toilet and the problem goes away.
Other commenters are right, run water in all fixtures to see if the smell goes away. If it does, the water is collecting in the elbows preventing sewer smell from coming in.
If the bath IS used regularly, then the odor may be coming from incorrect or poor drainage connections.