It could be about a strange, unknown creature lurking in the shadows. Maybe it's a creature that only reveals itself at night, and the two sentences describe the moment when someone first senses its presence and then realizes the danger it poses.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.