From the Super Mario Wiki
Retrieved from "https://www.mariowiki.com/index.php?title=508_Bathroom&oldid=2817323"
The toilet paper roll in the southeast corner is actually a roll of Bills. There is also a Gold Bar in the toilet. If Luigi approaches the contraption on the sink, it quickly starts up before fizzing out and smoking. There is a radio on the bathtub ledge Luigi can flash with the Strobulb to play different Luigi's Mansion medleys. One of Professor E. Gadd's lab coats can also be seen in the southwest corner, hanging above the radiator.
There is a drain in the middle of the floor Gooigi can fall through to get to a hidden area. He can then squeeze through the pipe in the southeast corner to get to a treasure chest with a Pearl inside.