Your septic system could be improperly vented. Your vent could be clogged. You could have a blockage in your sewer vent pipes. Sometimes birds make nests in these things. This could be dangerous, so call a plumber.
Sewer gas in most cases is methane gas. It smells very bad and and can be explosive. The plumbing fixtures in your home have traps installed between them and the sewer system to keep the sewer gas out of your house. the trap could be dry or a sewer leak is possible.
The water filled "trap" (curved pipe under a sink) on a plumbing fixture blocks sewer gas into the building. This trap could dry out in an unused fixture.
