Get the Guest Keycard first
Complete the Rebreather and Rope Gun routes, obtain the Shovel and dig Maintenance A for the Maintenance Keycard.
Use that card at the western Entertainment Bunker. Go through the gym, spa, atrium stairs and upper pool into the nightclub. Beyond the bar, the fourth seating alcove on the right has the Guest Keycard on its central table.
The Residential entrance uses the Guest Keycard directly. The Maintenance card is an earlier step, not the card to present at that entrance. Some older item listings confuse the two.
Enter the Residential Bunker
Find the opening in the rocks on the eastern shore of the large lake east of the mountains. Drop into the bunker passage and follow the stairs to the card reader. Use the Guest Keycard, then progress through the story sequence and hostile corridors.
Look for the Level 2 stand
Continue down to Level 2 and check the open suites on the right. Identify the traditional weapon stand and pick up the Katana. Guides count the rooms differently, so a rigid second/third-room number is less useful than the side, level and stand.
Verify Katana in the inventory. Before leaving, you can locate the surveillance control room and use its computer to open Luxury for the ending route.