A closed office door can mean concentration to one person and unavailability to another.

One signal can be read in opposite directions, and neither reader thinks they are interpreting. To the person inside, a closed door means an hour of uninterrupted work. To the person outside, particularly anyone already unsure of their welcome, it can mean not now and possibly not ever. Silence around a signal gets filled with whatever the receiver already fears.

If you use a signal like that, say plainly what it means and when it lifts; if you are on the other side of one, ask rather than settling for your own reading.