From the Super Mario Wiki
The Hidden Passageway is a location in Luigi's Mansion 3. It is found in the Tomb Suites on the tenth floor of The Last Resort, located near the very bottom of the pyramid. It connects to the Pit Chamber and Central Chamber.
To get to the hidden passageway, Luigi must locate and activate the statue at the bottom of the Pit Chamber, which causes sand to start pouring in from above. Luigi must use the Poltergust G-00 to blow sand to the right to reach the platform. At the start of the hidden passageway, there is a spider on the wall, three coins in the wall, and a coin on the ground. As Luigi heads up the ramp, he steps on a pressure plate which activates serpent statues ahead, which shoot poisonous arrows. Gooigi must walk past the arrows and make it to the chain, which he must pull to lift a wall in front of the statues, allowing Luigi to get past them. There is then a ladder Luigi must climb, with a few more spiders nearby. At the top of the ladder are two vases and an open sarcophagus. There are bats and treasure in the vases. Once Luigi investigates the sarcophagus, the lid closes, pushing him inside and transporting him to the Central Chamber.
Once Luigi escapes the hidden passageway, it cannot be revisited.