The first possible cause of a sewage smell in your bathroom is by far the easiest to fix. It is more likely to occur in bathrooms where fixtures haven’t been used recently, such as the sink in a guest bathroom or the tub in a bathroom with a separate shower. This is because the water that is supposed to sit inside the trap can evaporate over time.
If that’s the case, the solution is as simple as running some water to allow the P-trap to fill up again. The role of the P-Trap is to keep the sewer gases out of the home.