‘But supposing him to be in the company, they went a day's journey; and they sought for him among their kinsfolk and acquaintance, and when they found him not, they returned to Jerusalem, seeking for him.'
As both His parents clearly assumed that He was with them, presumably with his cousins, neither was worried until after a day's journey when they settled down to camp for the night, and at that stage were unable to find Him. But even then they were not too worried. They would think that He must be somewhere among their kinsfolk. It was only when they still could not find Him they must have realised with horror that He had been left behind, and have made straight back to Jerusalem, looking for Him.
Meanwhile Jesus continued to listen to the great teachers, and probably every now and then took a quick snooze in one of the porticoes of the Temple. He does not appear to have been worried, and possibly not even to have considered that He was being missed. After all His parents knew that He could be trusted. And His view was that surely if they had wanted Him they would have sought Him in the Temple, where they ought to know that He would be. (He could not conceive of anything else). He was still only a child, and was possibly not used to the Feasts, and the Temple may well have continued to be so crowded that He did not realise that the Feast was over. They were exciting days and He did not want to miss the opportunities they presented. This would serve to confirm that He knew that His parents would not be worrying when He did not go back to them at nights.