Swan left alone?

Hi everyone. I pass a pond daily that is in a field, so not accessed by so many people as not in a park for instance. There were two adult swans there, who last year produced 3 offspring. In the last week, one of the swans seems to have disappeared. No sign of it around the pond, no sign of nesting and the other swan has remained. I'm assuming they would not separate during the reproductive cycle, for nesting for example, and there doesn't seem to be any signs of attack. Where could the other swan be? Is the remaining swan safe or should it be reported? Thanks for any suggestions.