Harry looks to Mrs. Weasley who has made several comments on the fact that they will be alone. "Mrs. Weasley, you know I wouldn't do anything to Ginny. I love her, and I promise to treat her as if you were standing right there." Harry held his hands out to his sides to indicate no touching.
"I believe you dear," Molly said as she laid her hand on Harry's cheek, "I just want you to know I have a spell that will show me everywhere your hands have been on my daughter when you bring her home in the evenings." Ignoring the shocked look on both of their faces, she continued, "It doesn't matter what cleaning spells you use either, I can still see where your hands have been."
Harry and Ginny had known Molly was a great witch, she had been part of the Order after all, but to know something like that amazed them.