Secret Sleeper 

The Secret Sleeper is the extra person or people that stay in your hotel room on vacation, when the hotel believes only two are in the room.

Suzie, you're the Secret Sleeper sneak in through the back staircase to the room...here is a key.

Secret Sleeper by Hill Storm November 14, 2011

Secret sleeper 

Someone who has consumed a large amount of alcohol and suddenly leaves the room or party to pass out without notice.

That Luther is always a secret sleeper, disappearing in the middle of the party.

Secret sleeper by Modkrew16 December 28, 2013