This happens every year. Young Heron fledge, fly off, and come back. Usually it’s the young ones from the far island of this rookery. I have watched it before.
They will hunt along the waters edge where there is a supply of (almost) easy food.
They can see, and fly to, the old nests. Once in a while they do. It’s a comfort thing.
Now they will expand their territory, slowly going further away.
Many will come back next February and look around to make their own nests.