The swans may be sitting on a nest. The one swan may be foraging for food if it is the female, as she may be taking a break and then going back onto the nest. If it is the male, he will patrol the area and then return back to the nest to sit with the female. If this is the case, then you should have cygnets along with the parents, once again swimming around the pond in approximately 30-45 days.

If no swan returns, they may have found a better nesting/foraging habitat and are choosing this area to nest. There is no way to determine if something has happened unless you find a carcass. We know this is of no consolation, but wild swans, even captive swans in area where there is a lot of water and vegetation affords an enormous opportunity for the birds to hide or travel to another area. Predation would also be a major issue in these types of areas. The Regal Swan