Drain odours inside can indicate a dry trap, cracked air vent

https://online.seminolestate.edu/eportfolios/1186/home/why-you-need-a-professional-plumber-to-clear-blocked-drains

Drain odours inside can show a dry trap, cracked air vent, or partial blockage; Geelong properties with large trees or older sewer lines may require CCTV examinations to situate breaches and protect against reoccuring smells.