From the Super Mario Wiki
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At the start of the game, some of Princess Peach's luggage is blocking the door, preventing Luigi from exploring the bathroom. The next time he explores the RIP Suites, he can use the Suction Shot on the luggage to clear the door and explore the 501 Bathroom.
There is a bathtub, sink, and toilet in the 501 Bathroom. There is a Pearl in the toilet. If Luigi turns the bathtub faucet all the way, coins will come out of the tub. There is a stack of Bills behind some perfume bottles which can be seen through the mirror.