There could be several reasons. It is a young swan that is looking for a habitat for a potential mate to join it, or a swan that is used to the area and has lost its mate. Either way, if people are taking care of it by feeding it and it is in a safe habitat away from predators (wild and domestic, i.e., dogs, cats, humans, etc.), then it should stay and establish this pond as its forever home.

Its leaving during the winter months only to return during warm weather is a natural migration process. The Regal Swan