Are you absolutely sure you have a male and a female? If the swans are of the same gender, someone or something is chasing the other one out of the habitat. This could lead to the bird being killed by a predator, not good.

If he is a male, he may be out looking for another mate or to find his original mate, the mother to the daughter. Did something happen to her recently?

On another note, sometimes animals will go to a very secluded area to die if the are sick. You might want to get the swan to a veterinarian to ensure that there is nothing wrong. In any case, none of this behavior is normal or good for the swan. The Regal Swan