Why a physical signal beats a judgement
Judging from the corridor fails in both directions. A quiet room may hold a patient waiting alone. A room with a light on may have been left that way. A closed door may mean nothing at all in a practice where doors are always closed. Every one of those produces either an intrusion or a room that goes uncleaned for no reason, and both outcomes are avoidable with a signal that anyone can read.
The signal has to be positive rather than negative, meaning it indicates released rather than occupied. A room with nothing on the door is then treated as occupied by default, which is the safe direction to fail in. Systems built the other way round, where a marker indicates occupied, fail dangerously the moment somebody forgets to place one. Failing safe is the whole design principle here, and it is worth stating explicitly.
