After the Feast was over, while his parents were returning home, the boy Jesus stayed behind in Jerusalem, but they were unaware of it. Thinking he was in their company, they traveled on for a day.
Following the event, while his parents were returning, the boy Jesus remained in Jerusalem, but they were unaware of it. Assuming he was part of their group, they continued their journey for a day, which is similar to the situation where someone stays behind without being noticed.