If he couldn't reach you, he was responsible for being home. He is taking advantage of you. Use this to change the game. Apologize but say, 'let's be sure that you call me early in the day to ask if I can care for XXX, that way you'll know you have to be home if I have plans or you are unable to reach me. Also, if your door had been locked, I would not have driven away with XXX 'safely' in the house. My goal is to be sure both our children are safe.'
What's the big issue with the boy being at home alone??
Why would the wife apologize? It was her jerk husband who acted a fool.