It started innocently enough last winter, when the snow kept piling up and the whole building felt isolated from the world. I was 27, recently divorced, living alone in my quiet apartment. Across the hall lived Ethan—mid-30s, married, with a deep voice and a smile that always lingered a little too long when we passed each other in the elevator. His wife traveled a lot for work, and on those long, cold nights, I’d hear him moving around alone, just like me.