I think the term for what you have is a cesspool. A septic tank is part of a system with leach fields, and is normally self-contained sewage disposal system. A cesspool is a holding tank.
But, that doesn't really matter for your question! If that tub isn't used on a regular basis, the trap can dry out. If that happens, it becomes a direct path to the sewer. If running water (only need a pint or so) stops the smell, that's the issue. But, if the tub drain is not vented properly, it can get siphoned dry quickly, and make that happen as soon as something else drains, pulling that trap dry. If you can hear water draining while carefully listening at the (when you're not using it!), the trap is dry. A less likely thing could be if the trap has a hole in it, but that doesn't happen too often.
It's possible there's an accumulation of hair or other crud in the drain, and that's smelling.