Smell from unused bathroom
We renovated an old home and changed the entire plumbing almost a year ago, so all the pipes are new. Soon after we moved in, we started noticing a nasty smell from one of the bathrooms on the ground floor. This is a guest room and used quite infrequently. The smell comes and goes, and differs in intensity.
We have had guests stay in that room over several days, and the smell persists even when the bathroom is being used regularly, though the intensity reduces, and is strongest in the mornings before the bathroom has been used. There is no overflow that occurs, and water passes smoothly through the traps.
This drains from this bath are the last ones before the pipes get linked to the public sewer system in the street (there's at least 15-20 feet between them. The bath that's located right above this one (my daughter's room) very occasionally has an unpleasant smell, but it is different from this one, and I assumed it's because of lack of ventilation (she often forgets to operate the exhaust), but I don't know if it's linked. We don't have this issue in any of the other 2 baths in the house.
We haven't been able to identify if the smell is from the shower area or the toilet. We also don't know what the smell is of - it's nasty, but not a regular toilet or sewer smell. The plumber who laid out the lines has come in and investigated briefly - he says all the traps have water in them, and claims not to know where or what the problem is. Please help!!