I think she used the word 'kidnapping' because had she said 'missing', there may not have been the same amount of effort put into looking for Natalee.
Being 18 and an adult, even the police in the US don't consider someone as 'missing' until 24 hours.
The problem though is that there is nothing that we know of to indicate a kidnapping.
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those being the operative words. it has been made clear that they know things that they are not at liberty to divulge at this time for fear of entirely compromising the investigation.
You said:
"[i]The problem though is that there is nothing that we know of to indicate a kidnapping.[/i]
It is the absence of ANYTHING that indicates a kidnapping.