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.
A great two - sentence horror story is 'I heard my name whispered in the empty hallway. But I was alone in the house.' Just the act of hearing your name when you're supposed to be by yourself is really creepy. It gives the feeling that there's an unseen presence that knows you, and it could be anywhere in the seemingly empty house, waiting to do who - knows - what.
I think 'The clock stopped at midnight. Then I heard footsteps.' is quite scary. It gives this feeling of something ominous starting right at a spooky time.
Some two - sentence horror stories are truly spine - chilling. For example, 'I woke up to a noise in the kitchen. When I got there, all the knives were floating.' The simplicity of the setting and the unexpected, supernatural element make it effective. Another one could be 'She heard a baby crying in the attic. She doesn't have a baby.' It plays on our primal fears of the unknown and the unexpected in a very short space, creating a sense of unease and horror.