After several hours and no use, the bathroom accumulates some noticeable "sewer gas" smell, with kind of garlic. It is gone after a few minutes when the ventilating fan is turned on. The bathroom and house are new (circa 1990). There is a normal wood-frame townhouse apartment attached to an old brick house. There is no basement in the apartment. The floors start from ground level.
I think there could be a tiny air leak from the sewer tubes through poorly applied PVC cement. But I don't know if it is a common and easy to fix defect. I even tried to seal a few small holes in the drywall with paper and glue where the sewer tubes from the sink are going through the wall but it did not help.
What could be the cause of this smell?