Recommend a contractor trained in combustion safety and certified by one of several organizations. The Building Performance Institute and the National Comfort Institute come to mind. I personally feel that this is a serious problem and that all inspectors should refer similar circumstances to qualified, certified technicians in the field.

Combustion gases do not belong in the living environment under any circumstances. On their best day combustion gases are a problem, and on their worst day they they are deadly. Signs of spillage or backdrafting are a serious matter and should be reported accordingy.