I had the same question after finishing 'Stay Close' and went down a rabbit hole. The book is listed as a psychological thriller novel by Harlan Coben, so it's officially fiction. But Coben has a knack for pulling from real-life anxieties—suburban secrets, cold cases resurfacing, that feeling of a past identity haunting you—so it feels plausible. A quick check shows no major news case directly matching the triple plot of Megan/Cassie, Ray Levine, and Broome's investigation. That said, the disappearances and police corruption elements are, sadly, not uncommon in real life. It's fictionalized, but the emotional core—people trying to outrun their old selves—rings terrifyingly true.
I think that's why it hit so hard for me; the premise isn't a fantastical spy story, it's about choices made years ago tightening like a noose. The Netflix adaptation plays this up, making the suburban setting so ordinary it amplifies the dread. So, it’s fiction, but the kind that leans on a 'this could happen' vibe to unsettle you long after the last page.