Sewage system odours inside can show a completely dry trap, fractured air vent

https://share.relay.edu/eportfolios/217/tom-cruise/tips-on-keeping-your-plumbing-system-properly-maintained

Drain odours inside can show a completely dry catch, cracked vent, or partial clog; Geelong homes with big trees or older drain lines may need CCTV inspections to situate intrusions and stop reoccuring smells.