Well, Rosa’s room, then. The windows are said to be closed, not locked, but I’m just gonna assume they work like usual and can only be closed from the inside.
Still, with the key just somewhere on the floor, standard stuff comes to mind.
Someone else slipped in the key after the door was broken. If the whole group was inside the room, it could have been possible to pull off stealthily from the door, especially when they were busy lifting up Rosa.
Alternatively there’s always a loop of string through the gap in the door with the workbench as an anchor point, and then letting go of one end of the string midway through.