Take a look at this picture. Notice the boundary trap shaft. Do you have what looks like this in your yard near the sidewalk? If your home is more than 50 years old that is likely a whole house yard trap. These were installed in yards just upstream from your home's sewer connection with the municipality to prevent municipality sewer smell drifting into the home. Now, all fixtures in homes have p-traps directly below or, like toilets, built into the fixture. Yard traps can present a maintenence problem, as that is where clogs can often stop, and back-up the sewer in the home. Check with your plumber. It may time to remove that yard trap, and replace it with a standard clean-out.