Our major concern is that the swans are in a salt water lake. Most swans, unless they are the South American Black-Necked Swans, cannot eliminate salt and will not usually be able to survive in a saline environment. The one swan may have realized that this is not a good environment and left. Hopefully, the second swan will also leave and find a more suitable freshwater environment. If the first swan is nesting, the second swan should go to the nesting site. If this behavior is not observed, and the swan keeps milling in the same area, you might want to contact a wildlife/waterfowl rescue group and see if they can check out the area and possibly capture/rescue the milling swan and move him to a more suitable habitat. The Regal Swan