Kaoru could try to gain the trust of one of the kidnappers and then use that opportunity to escape. For example, if there's a less hardened kidnapper among them, she could pretend to befriend them and then make a run for it when they let their guard down.
Kaoru could try to escape by playing on the kidnappers' superstitions or fears. If she can figure out what scares them, she could use that to her advantage. For instance, if they're afraid of the dark, she could find a way to turn off the lights and create chaos to escape. Another way would be to try and find a weak point in the place where she's being held, like a loose window or a faulty door lock, and then use her wits and any available tools to break free.
Well, she might be held for ransom. Maybe the kidnappers think that as a nurse, she has access to valuable medical information or drugs that they can sell on the black market.
There could potentially be such a fanfiction, but it's not a very common or well - known trope in general fanfiction. It really depends on the fandom or the creative minds of individual fanfiction writers. Maybe it exists in a very niche or specialized area of fan - created works. You might also consider reaching out to fanfiction communities or forums to ask if anyone has heard of or read such a story.
It could be a former patient of Kaoru's who has some mental issues. He might have misinterpreted Kaoru's care as something more and when she didn't reciprocate in the way he wanted, he decided to kidnap her.
It could be a jealous coworker. Maybe they feel that Kaoru is getting all the attention or a promotion they wanted. Another possibility is a patient who has become obsessed with Kaoru during their treatment. Or perhaps it's someone from Kaoru's past who has a grudge against her.
One plot idea could be that the kidnapper has a personal vendetta against Kaoru. Maybe Kaoru witnessed something she shouldn't have, and the kidnapper wants to keep her quiet. Another idea is that it's a case of mistaken identity, and the real target was someone else in the same hospital. A third idea could be that a deranged fan of Kaoru's kidnaps her in an attempt to make her their own, thinking they can 'protect' her from the outside world.
Perhaps in this fanfiction, Kaoru is kidnapped because of her special skills as a nurse. There could be an underground group that needs her medical expertise for some illegal or secretive operation. So, they abduct her to force her to work for them.
Well, it might be that the kidnapper has some sort of medical need that they think Kaoru can fulfill. Since she's a nurse, they could be desperate for medical attention that they can't get through legal means. Or perhaps there's a villain in the fanfiction who just wants to create chaos in the hospital by taking one of its nurses, and Kaoru happened to be the unlucky one.
Kenshin would be extremely worried. He'd immediately start looking for her. He has a strong sense of justice and protectiveness towards Kaoru. So, he'd use his Battousai skills to track down the kidnappers as quickly as possible.
Philip could use his wits. He might pretend to be sick or injured to catch the kidnappers off - guard and then make a run for it.
One possible plot could be that a rival group of Kenshin's enemies kidnaps Kaoru to lure him into a trap. They might hold her in an old, abandoned building. Kenshin would then have to use his skills and enlist the help of his friends like Sanosuke to find her. The kidnappers could demand that Kenshin give up his sword in exchange for Kaoru's safety.