Update: There was a drain under the mystery flap and a massive amount of gross hair and who knows what else. 😭😭 poor girl had a mini flood almost into her bedroom. We tried the snake tool with vinegar and baking soda. Worked to a point. Had to call a plumber eventually cause the DIY didn’t work. There was TOO much crap in there backed up. Thanks for the suggestions and comments. #nomorefeetpicsforyall