There may be several reasons for the smell, which can occur inside or outside your home. If you notice an odor, it may be coming from a roof vent or other vent pipe that allows the system pressure to equalize. This is a normal part of your system. Sometimes these vents can become obstructed and clogged from leaves, debris, etc., or the vent pipe can freeze during prolonged cold spells. These situations could cause an odor inside or outside of your home. Another possibility is a down draft, or changes to wind patterns or other location-specific conditions, which can create an odor inside or outside your home. In these cases, the vent may need to be cleaned or raised.
If your drainfield is not working properly, that could be another reason you smell an odor inside your home or around the septic system. Soft, wet, or spongy soil (especially when there have been no significant rainfall events) around your drainfield is a good indication of a system failure. It is not possible to diagnose the exact cause of an odor remotely. EPA recommends you contact a local septic system service provider and/or plumber to address the issue.