Why did Joseph and Mary take one day's time to realize that Jesus had been left behind at the Temple? We read in   Luke 2: 43-46, the incident relating to the visit of child Jesus and his parents to the Temple  and his `getting lost' at Jerusalem: 


  After the festival 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. Then they began looking for him among their relatives and friends.  When they did not find him, they went back to Jerusalem to look for him.


The distance they would have to cover from Jerusalem to Nazareth was 65 miles and it would take them days together to complete the journey. Some traditions say (sorry, I am not able to quote any reference) that in those days, men and women took separate routes because of which each of the parents was unable to communicate with the other,  the presence of children with either of them. One is, however, unable to know why it took one day for Jesus'parents to realize that he  had been left behind. 
My question therefore,  is: who does the Catholic Church explain the time of one day that Jesus' parents  took to conclude that boy Jesus had been left behind at the Temple in Jerusalem.
Luke 2:43-44--and as they were returning, after spending the full number of days, the boy Jesus stayed behind in Jerusalem. But His parents were unaware of it, but supposed Him to be in the caravan, and went a day's journey; and they began looking for Him among their relatives and acquaintances.


As DJClayworth mentioned in his comment above, it may well have been assumed that he was with them There's no evidence that His parents even questioned His being in the caravan. He would have been among relatives and acquaintances and there would be very little reason for them to believe He was elsewhere.