How does the questioner atone for her sin after she caused a slight scratch to another car without the knowledge of its owner or her husband?

What befell the husband's car must be reported to him, and its value paid, or his forgiveness sought. As for the other unknown car, the damage should be estimated, and its value given as charity on behalf of its owner. If the owner is found, he has the choice between taking the money or upholding the charity.