What does Luke 2:45 mean?

The older and more responsible a child is, the less meticulous parents need to be about keeping track of them. Jesus, in this event, is nearly what His culture would consider an adult (Luke 2:42), on a yearly family trip (Luke 2:41), and travelling with friends and relatives from his hometown (Luke 2:44). He's also the only child in history to have ever lived a sinless life (Hebrews 4:15; 1 Peter 2:22). The thought of Mary and Joseph not tracking Him down until the end of a day of travel is not as negligent as it might seem at first.

As well, the time between when Jesus was left behind at the temple and His reunion with Joseph and Mary was not "a completely full three days," as in "72 total hours." The first day was the family's travel away from the city, the second was the travel back, and during the third day His family found Him speaking to teachers in the temple (Luke 2:46).