Adam came home when he heard that the doctor was ill, and probably dying. He approached the castle late at night, eager to avoid a confrontation with the townspeople, or worse, his father’s servants.
As he approached, he wondered what he was even doing there. The old bastard wouldn’t appreciate his presence, and even after being gone for years, he had no affection for the old man. But he looked down at his scarred and twisted hands ant thought that maybe, in the end, we are responsible for our creators.
Thus it was that The Creature returned to Castle Frankenstein.