This might take a couple of runs until you finally get the chance to be called down into a basement door that appears in the fridge room.
Once you get called into this event, it will take you down to a secret basement.
Once in there, you will encounter quite a few frozen demons which you will have to open their mouths when interacting with them.
Some of them will have a number and a position for the number. Remember this number and the order of each digit, this will be needed when you get out.
When the event is over, you can go to the cabinet next the the tubing pipes, there is a small safe which requires a combination. Enter the 6 digits you got from the Demons. This will give you a key.
With this key you will be allowed to open the basement door.
EXTRA: Take the old large key that is in the drawer under the papers, this will unlock a secret achievement when you open this second door in the basement. Unlocks "Our Secret Below"