Drain odours inside can show a dry trap, broken vent

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

Sewage system odours inside can show a completely dry trap, cracked air vent, or partial obstruction; Geelong buildings with huge trees or older drain lines may need CCTV examinations to situate intrusions and avoid reoccuring smells.