We have occasional sewer odor on the second floor of our two story home. We had a plumber check it but had no success. Odor is not constant, only on the second floor and doesn't correlate with plumbing use (coming home from work after hours of no one in the home to use plumbing, odor can be as strong as times when family is present)

Smell could be coming from the vent pipe exiting the roof?