Mary is staying at the Chic Shell Hotel. But they have a policy that you can't use the elevator unless you have a key into a room.
This is the low-spoiler guide for "Accessing the hotel elevator" puzzle in Voodoo Detective. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.