Drain odours inside can suggest a completely dry trap, fractured air vent

https://canvas.pasadena.edu/eportfolios/4924~6653/home/common-plumbing-leaks-that-you-may-experience-in-your-home

Drain odours inside can show a completely dry catch, split vent, or partial blockage; Geelong properties with huge trees or older drain lines might need CCTV examinations to situate breaches and protect against repeating smells.