How did the creature learn that the cottagers had moved out of their cottage?

With the arrival of morning, the creature allows himself to hope that all is not lost: perhaps he can still endear himself to the elder De Lacey, and thereby make peace with his children. When he returns to the cottage, however, he finds it empty. He waits, tortured by anxiety, until Felix finally appears in the company of a strange man. From their conversation, he learns that the De Laceys have determined to leave the cottage out of fear that he (the creature) will return.