^ I know that particular poster disagreed with you earlier, but there's no need for the snippy attitude in my opinion; it's a reasonable question.
I feel that JMB was simply covering up the fact that, CB pretty much ran away that day, for all intents and purposes; and it was directly because of JMB. I can't confirm this, obviously, but all the signs point to such. He was whipped that very day at 5:30 when JMB found him on his skateboard in the middle of the street, then told to clean the car port. When JMB discovered he, once again, directly disobeyed him after already being punished once, he felt obligated to find him. The punishment, and the fact he was disobeyed a second time, fueled the urgency to find CB that day.
Posey, a witness who said JMB came by his house looking for CB around 7, said that JMB stated, "Well, I'll just have to give him another whooping" (paraphrasing).
I don't mean to imply that this directly proves JMB's involvement in the crime; just answering the original question of, why was it so crucial to find CB on this particular day for JMB (if the boy had stayed out late before); and again, the answer is, because he directly disobeyed JMB that day and broke his initial punishment/grounding (i.e. clean the carport). If you read through the various accounts of JMB, you can almost tell he's embarrassed and somewhat reluctant to admit that he had punished and whipped CB that day -- not only that, but obviously, it looks all the more bad/incriminating considering would transpire that day.