Sewer odours inside can show a completely dry trap, cracked vent

https://academy.educause.edu/eportfolios/12410/home/reasons-why-homeowners-trust-plumbers

Sewage system odours inside can indicate a dry trap, cracked air vent, or partial blockage; Geelong residential properties with huge trees or older sewer lines might require CCTV inspections to find intrusions and prevent recurring scents.