may be coming out your vent on your roof. as air cools
outside,pressure drops and the higher warmer pressure inside sewer
system is forced up and out,then cools and comes down to ground level .
i never had this problem on septic tank,but do now hooked onto sewer
system. i extended my vent pipe up about 4 feet in hopes the gases would
tend to blow away farther, they do somewhat,if its windy .. lucas