From the Super Mario Wiki
The Security Office is a location in Luigi's Mansion 3. It is found in the Hotel Shops on the third floor of The Last Resort. It is east of all the shops and right next to the Lost and Found. When Luigi first enters the shopping area, he spots Kruller patrolling the area. Kruller investigates a vending machine outside the security office, then enters the room, dropping the red diamond key needed to open the cash register in the jewelry store. Gooigi must open the cash register in the barber shop to get the star key needed to unlock the door to the security office.
The blue gem is found in the vending machine next to the security office. Luigi will need to flash the light with the Strobulb a few times for the gem to pop out. If Luigi peeks through the window, he will see Kruller at his desk holding the fourth floor elevator button. As soon as Luigi enters the security office, Kruller sees him and puts on shades and grabs a water pistol from the Lost and Found. To battle Kruller, Gooigi must slip between bars between the security office and the Lost and Found, suck up Kruller's sunglasses, then suck him up. After defeating Kruller, Gooigi recovers the third floor elevator button.
There is little more in the security office other than a desk, cabinets, shelves, and a locker. Luigi can use the Suction Shot to open the locker. There is also a security footage area in the northwest corner showing footage of the shops.