Well, it depends on the context of the fanfiction. Hermione is a very inquisitive character. So when she discovers the cupboard, she might immediately search for clues. It could be that the cupboard contains something that has to do with a mystery she's been trying to solve, like a hidden spell book or a clue about a missing student.
The fanfiction could take many directions. Hermione finding out about the cupboard could be the start of a big adventure. She might find that the cupboard is magically protected and has to use all her knowledge and skills to break through the protection. Once inside, she could discover a whole new world of magic that no one else knew about. This could lead to her having to make some difficult decisions about whether to share this discovery with others or keep it a secret for the safety of Hogwarts.
In this fanfiction, Hermione might be shocked when she finds out about the cupboard. She could start an investigation to figure out what's been going on in there. Maybe it's a hidden place full of secrets related to Hogwarts' history.
He could be furious. In many fanfictions, Snape is a strict character. Discovering something about a cupboard might lead him to suspect some sort of rule - breaking or hidden secrets. He might start an investigation right away to find out what's really going on.
There could also be some unknown characters in the fanfiction. Maybe a secret society within Hogwarts that has been using the cupboard for their own purposes. When Hermione finds out about it, she gets caught up in their affairs. These unknown characters could be students from different houses or even former Hogwarts students who have come back for some hidden agenda related to the cupboard.
She might be really surprised. Fanfiction can be quite different from the original story, and if Ada is used to the official Soukoku storyline, seeing all the creative and sometimes wild interpretations in fanfiction could be a shock.
Tony might be really surprised. He could have a range of reactions from being amused to being a bit uncomfortable, depending on what the fanfiction is about.
She might be initially shocked. If the fanfiction contains some untrue or inappropriate things about Clarke, Abby could be protective and try to find a way to stop it from spreading.
Tony could be really surprised. Maybe he'll first be curious to read it and see what others have imagined about Skip Westcott in this fanfiction.
In this kind of fanfiction, Cedric might feel shocked and angry. He could decide to protect Harry from further harm. Maybe he'll try to find the abuser and put a stop to it.
Well, Dudley might be jealous at first. He's always been used to being the 'big shot' in his own little world. If he finds out Harry is famous in fanfiction, he could start to feel overshadowed all over again. Maybe he'll try to downplay it, saying it's just some silly made - up stories.
Charlie might be shocked and filled with anger. He could immediately seek to comfort Harry and ensure his safety. Maybe he'll vow to find the perpetrator within the context of the fanfiction's story.
In some fanfictions, Hermione might find out she is a Malfoy due to a spell gone wrong. For example, a spell that was meant to reveal family lineages got mixed up and suddenly she has a connection to the Malfoy family in the magical world created by the fanfiction writer. It could also be a result of a hidden prophecy that links her to the Malfoys in an unexpected way.