I'm waiting to see if anyone brings up 'The Last Time I Lied'. The summer camp setting isn't a locked room per se, but it functions like one. A closed community, a group of girls who vanish into the woods at night, and a protagonist returning years later to a place that feels frozen in time.
The camp, especially during a storm when everyone is gathered, becomes an isolated bubble. The mystery hinges on secrets kept within that small, insular world. It's less rigid than a single house, but it has that 'closed circle of suspects' feeling. The lake, the woods, the cabins—they create a natural boundary that makes the disappearances feel both impossible and inevitable. It's a different flavor of confinement.