If it isn't coming from inside the house then it is coming from outside. What I mean by that is the city sewer is backing up into the house. If what you are saying is correct in that no additional water has been introduced from inside the house I would investigate the city sewer. You can check if other neighbors have the same problem.
Any rains lately? Sometimes older citys have combined storm and sanitary sewers that become overwhelmed in heavy rains and ends up backing up into peoples basements, especially when there is a bathroom in that lower level.