The Last Riddle

At 1 AM, I was home alone, scrolling through a riddle app on my phone in bed. A new puzzle popped up: *'It is 1 AM. A man is home alone. He feels someone standing behind him. He turns around three times, but sees absolutely nothing.'* I stared at the screen, unable to figure out the answer. Right then, I felt it—the distinct sensation of someone standing directly behind me. I slowly turned my head. Nothing. I faced forward, and the feeling returned. I checked again. Empty. The third time, the presence felt so close it was practically touching my spine. I whipped around, but the room was still completely empty. I didn't dare to turn around again. Yet, I could clearly feel something pressing against my back, perfectly motionless. I laid there paralyzed for what felt like hours, before finally mustering the courage to turn to the other side. It was empty. But on the pillow next to me, there was a deep, fresh indentation. As if someone had just been resting their head there. I reached out to touch it. It was ice-cold. Beside it lay a single, long strand of hair. It definitely wasn't mine. The next morning, I was found dead in my bed.