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

http://photohistory.oregonstate.edu/works/half/the-importance-of-regular-plumbing-maintenance

Drain odours inside can indicate a dry trap, split vent, or partial obstruction; Geelong buildings with big trees or older sewer lines might require CCTV assessments to locate invasions and protect against persisting scents.