also have to note the following that may shed some light:
The basement is a walkout (with very high ceilings) and is not finished so I can see the floor underneath that bath when I go to the basement - nothing looks funky underneath.
Have smelled sewage once before in the spring but in the BASEMENT - trap on the other side of the house dried out and we noticed it (strong smell, much stronger than current odor) after doing large amounts of laundry.