One of my favorite suspense tropes – someone vanishes and no one knows where they are, or who took them. Sometimes it’s a kidnapping, other times it’s that the character had to escape, which often crosses over with forced proximity and/or hate to love!
In the Montana Series I wrote book 1 and sudddenly there were two young man who’d gone missing, and it informed a lot of character reaction to situations in book 1. Then in book 2, there is the cool amnesia theme, and in book 3… you’d need to read them to see. There is also the Lancaster Falls series, where the trilogy begins with a missing boy, Casey McGuire, and soon becomes much more than that. Three romances, one over arching story, and three books, with the final one due in a couple of weeks.