In the 'Castle thinks Beckett is cheating' fanfic, what could be the reasons for Castle's false suspicion?

It could be that Castle is overprotective. He loves Alexis so much that even the slightest sign of Beckett not being on the same page as Alexis in Castle's eyes makes him think Beckett hurt her. Maybe Beckett gave Alexis some career advice that Castle thought was too pushy.

Maybe in the fanfic, Beckett got a new job opportunity in a far - away place. This could make Castle think she's leaving him.

She is probably angry and frustrated. She would demand Castle to explain why he thinks so and would be hurt that he doesn't trust her.

He might be in denial at first. He could keep trying to convince himself that there's no way Beckett would leave him.

One possible reason could be that they are on a secret mission. Maybe they are trying to protect a key witness or important evidence from a powerful and dangerous criminal organization. In the world of fanfic, this could lead to them having to disappear to keep their whereabouts unknown.

Maybe he's afraid of losing her because she's his partner and they've been through so much together. Their relationship is very important to him.

In the fanfic, it could be that there are signs that Beckett is hiding something. Perhaps he has unexplained absences. He might say he's at work but Alexis can't reach him during those times. Also, he could be overly protective of his phone or personal space, which makes Alexis suspect that he's cheating.

Jealousy could also play a part. Castle is often surrounded by beautiful and adoring fans as a famous writer. Beckett might start to feel insecure, especially if she sees some of these fans getting a bit too close to Castle. On the flip side, Beckett's dedication to her work and her male partners in the police force could make Castle jealous, ultimately leading to their break - up.

Maybe they stumble upon a government conspiracy. The government wants to keep something hidden and they are seen as a threat, so they are made to disappear. It could also be related to a scientific experiment gone wrong. They are in the wrong place at the wrong time and get caught in an energy field that transports them to an unknown location, causing them to disappear from their normal world.

In the fanfic, it could be that rumors start to spread around about Castle being unfaithful. Beckett might be influenced by these rumors, especially if they seem to come from reliable sources within the story. Also, if Castle has been acting strange lately, like being secretive about his whereabouts or his phone, that could make Beckett suspect that he's cheating.