Where are you now? - Mary Higgins Clark

It has been ten years since 21-year-old Kevin MacKenzie, Jr. ('Mac') went missing. A Columbia University senior, about to graduate and already enrolled in Duke University Law School, he walked out of his room and has never been seen again. However, he does ritually call to his mother every year: on her birthday, on his birthday, and on Mother's Day. Each time, he assures her he is fine, refuses to answer her frantic questions, then hangs up. Even the death of his father, a corporate lawyer, on 9/11 does not bring him home, or break the pattern of his calls.
Mac's sister, Carolyn, is now 26, realises that neither she nor her mother will ever be able to have closure and get on with their lives until they find her brother. She sets out to discover what happened to Mac, and why he has found it necessary to hide from them.
Her journey into the world of people who willingly disappear from their own lives leads her to learn about others who may or may not still be alive, and ultimately to a deadly confrontation with someone close to her who suddenly becomes an enemy - and cannot allow her to disclose his secret . . .