It could be that she leaves at night to avoid being seen by humans. She might be exploring the world outside her home which she couldn't do during the day due to the fear of discovery. There could be other werewolves or supernatural beings she is trying to meet in secret.
One possible plot could be that she has a secret mission. As a werewolf, she might be using the cover of night to protect her family or friends from some kind of danger. She could be hunting down a villain or a threat that exists only in the night world of the supernatural.
It could also be that there is an older werewolf, like a mentor figure, who is important to the werewolf woman. This mentor could be guiding her from the shadows, and the woman's nightly outings could be part of her training or her search for something important to the werewolf community.
In some fanfictions, he might leave due to a love interest. He could meet someone who makes him question his vows to the Night's Watch. This person could be from a different region or have a special connection to him. For example, she might be a noblewoman with a hidden agenda, but Jon is drawn to her and decides to leave the Night's Watch to be with her and get involved in the political intrigue of the Seven Kingdoms.
It could be that she's in danger or being attacked. Maybe she saw something terrifying.
Maybe Percy leaves the hunt because he has some personal issues to deal with. For example, he might have a family matter that requires his full attention. Or perhaps he has a different mission or goal in mind that conflicts with the hunt.
It might be about a woman who mysteriously vanishes from her local area. Maybe she was involved in some sort of local intrigue or had personal secrets that led to her disappearance.
The plot could involve the night woman going on a journey through the city at night. She might encounter different people and situations that change her perspective. For example, she could meet a homeless person and learn about their struggles, which makes her more empathetic.
One possibility is that Harry leaves Hogwarts to go on a solo adventure to find more about his family's past. He might travel to different wizarding communities that his parents were associated with, uncovering secrets along the way.
Perhaps Steve leaves because of a personal sacrifice. He might discover that his presence is putting Tony and Peter in danger, like there's a villain targeting those close to him. So, to keep them safe, he leaves without much explanation. It could also be that he has some unfinished business from his past that he has to attend to alone, which forces him to part ways with Tony and Peter.
One possible plot could be that Harry leaves Hogwarts to hunt down the remaining Death Eaters on his own. He might feel that he can't put his friends in danger anymore and takes it upon himself to rid the wizarding world of the last remnants of Voldemort's followers.
It could be about a character's unexpected adventure during one night and two days. For example, a fanfic of a superhero who has to deal with a new villain that emerges during this short time frame. The superhero might struggle at first but then discovers the villain's weakness and saves the day.