From the systems security mindset - the doors and the windows are only the most obvious ways to enter the room, not the only ones. If you secure them very well,

From the systems security mindset - the doors and the windows are only the most obvious ways to enter the room, not the only ones. If you secure them very well, then the walls, floor and the ceiling become the best points of entry. (Chimneys are kind of obvious) Maybe the soon-to-be murderer just lifted the ceiling with a crane, killed the victim (thus becoming the murderer) and put the ceiling back in its place?