Perhaps one of the more common reasons for sewer gas entering the house is because plumbing fixtures are drying out. You will likely experience this with bathrooms that are not used very often or during periods of dry weather. Plumbing traps are one example of this. They are designed with a water barrier that stops sewer gas from coming back up into the home, if they start to dry out there is no longer a barrier and so gases will enter the house.
Sewer gases can also enter the house from cracks that have started forming in ventilation pipes, plumbing drains or foundations. If the home has been built with plumbing vents positioned too close to the home's air intakes, you can also experience sewer gas smells inside the home. Another reason could simply be the way air flows around the building, causing the smell to enter indoors.
Is a sewer gas smell a cause for concern? This will really depend on the reason sewer gas is leaking into your home, so it is very important that you have the problem diagnosed by someone qualified. Sewer gas can result in some very serious problems such as sulfide poisoning, asphyxiation from sewer gases displacing the homes oxygen and risk of explosion and fire.