Here is the info we sent to Janice:

If something or someone (predators) scared the family, they will seek a safer habitat for the swans. They may have taken a stream or other waterway to another area for not only protection but better food resources. You might check with the local RSPCA or Swan Support in Datchet (since we're not sure where you are located) and see if they may be able to advise if there was a swan rescue in your area. If there was, the family may be in treatment and released back to your area as soon as possible.

In your case Lexi, your family of swans may have been chased by the other swans or your swans left the area for the same reason as above. Swans will usually stay in their same habitat, but territorials skirmishes do occur and swans will move accordingly. The Regal Swan