In Gillette Castle at Gillette Castle State Park in East Haddam, CT.
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.