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501 Balcony is a location in Luigi's Mansion 3. It is found on the fifth floor of The Last Resort, the RIP Suites. It is a balcony adjacent to Room 501, Princess Peach's hotel room. The red Gem can be found here.
Luigi cannot explore the 501 Balcony at the beginning of the game, as the door leading to the balcony in the northeast corner of the hotel room is invisible and is blocked by some of Princess Peach's luggage. When he explores the RIP Suites later on, he can use the Suction Shot on the luggage and use the Dark-Light Device to reveal the door, then walk out onto the balcony.
There are plants and benches out on the 501 Balcony. There is also a watering can and a table with a cup of coffee and a slice of cake. Luigi can use the watering can to water the small plants in the northwest and southwest corners, then flash them with the Strobulb to get treasure, similarly to in Luigi's Mansion: Dark Moon.
The red gem can be found to the right of the 501 Balcony. Luigi needs to follow the trail of coins along the ledge to the gargoyle, then simply take the gem from its mouth.