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502 Bathroom is a location in Luigi's Mansion 3. It is found in the RIP Suites on the fifth floor of The Last Resort. It is a bathroom adjacent to Room 502, Mario's hotel room. The yellow Gem can be found here.
At the start of the game, a cart 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 cart to clear the door and explore the 502 Bathroom.
There is a bathtub, sink, and toilet in the 502 Bathroom. There is also a drain in the floor which Gooigi can fall through. He can then use a Suction Shot to pull off the wood barring the valve, then turn the valve all the way to stop the flowing water preventing him from getting to a treasure chest and redirect it to the bathtub. Gooigi is then able to open the treasure chest.
Luigi is also able to turn the bathtub faucet all the way to fill the tub with water, lifting a suitcase out of it which he can use the Suction Shot on to get the yellow gem.