The dust outline shows where an absent object continued affecting the room.

Removal leaves a shape. The object is gone and the room still shows exactly where it stood, in the clean rectangle nothing else could reach, so the absence keeps a presence for a long time afterwards. What has left continues to organize the space around it.

Expect this after any departure, a person, a job, a routine that ended. The outline is normal and it is not a sign that you have failed to move on. You can leave it for a while, and when you are ready, the way it fades is not by being scrubbed but by something else gradually being placed there.