Sewage system odours inside can suggest a dry catch, 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 completely dry trap, split vent, or partial blockage; Geelong buildings with large trees or older sewer lines may need CCTV evaluations to find intrusions and prevent repeating smells.