Dual Cell (2008)

A group of friends decide to take a late night road trip. Along the way one of the girls needs to make a stop. A lone gas station sign shines in the darkness. She goes in to find a restroom. The only clean stall is occupied or at least it seems so. As she waits her friends call to get her to hurry, but she is determined to wait it out, especially after it seems that the person in the stall has mysterious and mischievous ways. Her curiosity about the stall dweller eventually turns to frustration, and she ends up confronting her fear and discovering the truth contained in the dark cell. The intrigue that draws her in could keep her there permanently.