Trap (2013)

One day, a woman feels a stranger staking her when she was exercising in the park. She nervously gets off from the exercising equipment and looks around, but there is no one in the park. She gets on the elevator anxiously, but the man in the park suddenly takes the elevator on the 3rd floor! She gets off hurriedly and runs away from him, but the man follows her. As soon as she goes into the house to call 911, the man is gone. As she feels very unstable and uncomfortable looking around the lecture hall, she catches the man sitting in front! He glimpses at her and walks out, implying her to follow him outside. However, no one seems to notice his presence. Who is this guy...?