The empty swing moves differently after the child who used it has gone home.

What you have just seen changes what you are looking at. A swing moving on its own is only physics, and for a few minutes after somebody has gone it carries them, because your mind still holds the scene it watched. Places keep a short afterglow of the people who were in them. That is why leaving a room can feel like an event.

If a small absence catches you off guard, let it be ordinary rather than a sign of anything. Notice that the ache tends to match how good the presence was. And take it as a reason to be somewhere fully while you are in it.