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What do you think Kim felt had happened to Fiona Cowley? Why?

Kim thinks she has been abducted because she 1. hasn’t been seen in 3 days at the hospital visiting her brother and 2. Kim found her pet gerbal whose water bottle and food bowl was empty and Fiona would never let her pet go without food or water.

I’d say Kim thinks like I do, that Fiona has been abducted, probably by someone at the Preece place.
Why? As well as the reasons given by Eadie and John, >2 EadieB:, Fiona would not have left her father and brother to cope with the current situation on their own; and she was last seen heading towards the Preece mansion.

I think she's been taken to protect the secret they are all sharing.

i agree that she seems likely to be abducted. The Cowley family know too much and whoever is behind this atrocity(and the Preece family seem the likeliest perpetrators) needs to silence the cleverest member of that family.