Drain odours inside can suggest a completely dry trap, cracked vent

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

Drain odours inside can indicate a dry trap, split air vent, or partial obstruction; Geelong buildings with large trees or older sewage system lines may need CCTV inspections to find invasions and protect against repeating scents.