In Gillette Castle in East Haddam, CT. This was used as a guest bedroom, and Helen Hayes stayed here as a young girl.
Secret passageways in mansions and castles are an old dramatic staple, so of course Gillette Castle would have them. This passage leads from the Study to the foyer, where William Gillette would suddenly appear at the top of the stairs to greet guests. He would also use it to sneak out of the castle if he had any visitors he wanted to avoid. He had a network of mirrors throughout the castle he would use to see who was in Great Room before meeting them, or not meeting them.
This lower room was used by the builders during the construction of the castle, and later as a workshop where Gillette designed and made the theatrical elements of his home. It’s just off the entry hall and was not apparent to his guests. It’s behind glass so the public won’t disturb the original items in the room.
On a personal note, a branch of my family helped build the castle in the 1910s, according to my father. I haven’t been able to find any reference to them, though. Their name was Bannon, if anyone comes across a reference they can point me to.