Yesterday I could handle the smell no longer. It was just unbearable. So I bravely removed door-like thing. I will not post a picture of the disgusting mess that was on the other side.
When the addition was built and the bathroom was added, whoever did the plumbing work never screwed in all the pipes properly. Basically pipes were just sitting on top of one another. As a result, a bit of whatever went down the tub drain collected on the foundation underneath my house. Nice. I can't even begin to imagine the mold issues that this did cause or would have caused had we not caught it and it continued to drip for eternity. In a strange way, it's actually a good thing that Isabelle started using the tub as her own personal bathroom, because the smell of her urine collecting under it alerted us to the fact that something was wrong. Thank you Isabelle.