Neighborhood Watch (2013)

On a seemingly ordinary day, Min-Sook (Kim Min-Joo) goes to her car to leave for work. There, she finds a blue bag hanging on the car's side mirror. Min-Sook looks inside the blue bag and food trash falls out. She's freaked out by that, but thinks Soo-Ji (Kim Yoo-Hyun) is playing a joke. The next day, Soo-Ji finds a blue bag in front of her apartment door. Min-Sook's husband Hee-Chan (Ki Tae-Young), who is an unpopular novelist, views it as a misunderstanding or jealousy between his wife and Soo-Ji. Blue bags are then found around other residents in the apartment complex.