Sewage system odours inside can suggest a completely dry trap, split vent

https://www.pitchero.com/clubs/eyelsh/news/common-plumbing-leaks-that-you-may-experience-2923600.html

Drain odours inside can indicate a completely dry trap, broken vent, or partial obstruction; Geelong residential properties with large trees or older sewer lines may need CCTV assessments to find invasions and avoid repeating smells.