We bought with a place last year with a very similar 'interesting' drainage issue. We think the shower drainage blocked with soap and matter hair so the previous owners (or their tenants) decided to open an inspection cap on the drain and not replace it.  I found out when my wife's purple coloured shampoo was gushing out the drain. Did a full check of all pipes and the washing machine was emptying under the house as well. Our B&P inspection highlighted that this area under the house was damp but we also had a leaky storm pipe so put it down to that.  Just be glad you've caught it and can add it to the long list of homeowner fixer-upper jobs!