And I was, at first. Over the next few days, life returned to normal—it was a relief to not have to stress about my marriage anymore. I saw no other signs of Tanner, and in just over a week, he'd be off to college. That should be the end of it.

But then I remembered Tanner's texts. I knew it was silly to worry about, since clearly Amanda hadn't been responding to him, but I couldn't let it go. How did he get her number? And what did they say to each other?

I never solved the first mystery, but I developed a couple theories. They weren't connected on social...