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Koi do not hibernate in the traditional sense of the word but they do move to the bottom of the pond where the water is warmer and are much less active due to the overall colder water and air temperature.
So technically Koi do not hibernate because the definition of hibernate would entail a period of sleep, profound sleep.
You are not seeing the Koi because they most likely will not come to the surface until the water warms up.
Best wishes and please let me know if you have any questions.