All sewer (methane) gas from septic or sewer systems vents through the roof.
Higher hills, trees, buildings, etc. around your home, and/or the prevailing breezes, can cause an inversion from time to time that keeps the sewer gas down where it can be smelled instead of "blown away".
One solution might be a higher vent pipe(s). Two feet higher might make the difference in your particular case. You'll just have to experiment.
If your vents are plastic (black ABS or white PVC), it will be very easy to glue on a collar and extension pipe.