What does the room contain? If it’s a valuable artifact, surround the room with rooms with mundane objects, and build the room around the valuable artifact without any doors or windows.

If the room contains some kind of social circle or government function (think the 5 elder stars in One Piece), I think you’d again try to make the room seem unremarkable, but this time post a guard INSIDE the door. Possibly with an antechamber between the actual room and the hallway so that it seems like there’s just a guard in an empty room. Maybe you post many guards there and make it look like its their off duty room where they just hang out