we’re doing a 1 year builder warranty inspection and the customer had neighbors with similar sewage smell coming from the master toilet at times. the home builder repaired some leaks in the waste lines and this seemed to fix the problem. can someone explain to me how these smells are getting back into the bathroom through the water closet so I can explain it to my customer in detail?
The mounting flange, toilet ring (wax seal).
Maybe the “masters” in this community eat well… 
What leaks? Where?
Yup. Builders love using the cheapest materials available (can you say profits?), especially with materials that are not readily visible. Simply, cheap wax rings are just crap (pun intended), and leak after a short while, if not immediately, from rushed and haphazard installation.
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Sewer gasses enter the house through the drainage pipes, where exactly? nobody on this forum would know, get a plumber with a brain out there to figure it out. Smoke test may be necessary. Before you do so, make sure all traps are filled with water and smell still persists.
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Good questions… I’m trying to lock it down. Info coming through multiple people
- evaporation of water in the P-trap piping.
- broken seal around the toilet in the wax ring or the caulk.
- A burst pipe.
- tree’s roots have grown into or have caused damage to your sewer pipes.
- the sewer or main drain has bellied, collapsed, deformed, or deteriorated.