The Mysterious Room is a location in the roof of the attic in The Ducks' lair. It only appears in the episode "The Mystery of the Poster". Don is figuring out the suspect/mystery to find out who covered his Next HarGreeny poster in ketchup. In the last scene of the episode, Guin told them that he bumped to Flick who is eating her taco. He tried to clean up Don's poster which it didn't come off. But instead, he just wrote the sentences "smarty pants", and ran away before Don see it.
- According to the dictionary the Mysterious Room is a secret room which is in the roof of the attic. It was used for Don and the others to figure out who covered his poster in ketchup.
- No, the room is not for storing old things, it's used for them to figure out mysteries and clues.