Minnows really do not go anywhere else in the winter but they can be harder to find during freezing cold weather when the pond or lake is frozen over.
When the pond or lakes start to freeze over the minnows usually are near the shorelines and as the winter progresses and the O2 levels drop, they go deeper into the lake or ponds.
So they could be more towards the bottom of the pond or lake which is why they are hard to find in the cold weather especially when the lake or pond is frozen.
Basically when the winter comes the minnows go three places under the ice, they hang right near shore at the mouths of small incoming creeks, they hide in vegetation, or they hide in the spaces between big rocks.
So the minnows are still there in the water in the winter but they are just farther in the lake and pond.