Sewage system odours inside can show a completely dry catch, broken vent

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

Sewage system odours inside can indicate a dry trap, broken air vent, or partial obstruction; Geelong homes with huge trees or older sewer lines might require CCTV evaluations to locate intrusions and stop repeating smells.