Every fixture has a trap holding a water seal that blocks sewer gas. In a guest bathroom or utility that goes unused, that water evaporates and the seal is lost. Run each tap for 30 seconds a month and the problem disappears.
If all traps are wet and the smell persists, look at the soil vent pipe — a blocked vent causes traps to siphon out when other fixtures discharge, which is why the smell often follows a flush.
A persistent outdoor smell near a wall or path, especially with damp ground or rat activity, suggests a cracked drain and warrants a CCTV survey.