From the Super Mario Wiki
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At the start of the game, the 503 Bathroom, much like the rest of the hotel and Room 503, is brightly lit up and colorful. There is a bathtub, sink, and toilet. When night falls upon The Last Resort, the 503 Bathroom becomes dark and gloomy, much like the rest of the hotel. There is now a mouse hole in the northeast corner of the bathroom, there is a thorny plant above the toilet, the gift basket is replaced with a basket of bones, and the toilet has a claw come out of it when Luigi opens the lid. Four invisible Gold Bars can be seen through the mirror. Luigi can simply use the Dark-Light Device on them to reveal them, then collect them.