Swans can stay together for life, but if someone comes along that is better, they will re-pair. Pairing is usually dependent upon successful breeding and raising of young. So, usually swans mate for life, but not always.

Swans will also pair with swans of the same gender if opposite sex swans are not available.

More than likely, the swans took off for the migratory season and/or to find another habitat/mate. Once the birds return on the migratory path or if they are unsuccessful in finding another mate/habitat, the birds may return. For this time of year, this behavior is normal. The Regal Swan