The swans may have built another nest in another area of the pond. If something scared the birds, predators, people or just not a good area, they will rebuild. This would explain why you do not see the other bird. Of course, the other bird could have died. If this pair has been around for many years and successfully produced cygnets, it is more likely that something did happen to the mate as they tend to stick with the same mate if they have successfully produced cygnets for several years. Are you sure it is the same pair that you have seen in previous years? If not, they may have been juvenile swans looking for a habitat and the other flew in another direction to pair up with another swan.

Finally, the behavior you are describing regarding the rubbing of the body and dipping the head is bathing. The birds will oil their bodies by rubbing their heads and necks along the uropygial oil gland (preen gland) located on the rump of the bird. This oil is necessary for the health of the feathers, water repellency and the zipping of the feathers. The Regal Swan