I have seen many times bees fanning in upper entrance in -5C but normally when we have -25c, it is February and hives are under snow. In -25C the upper entrance is closed with mouldy frost.
One reason to loud hive is that they have a mouse inside or strong odor of mouse piss.
But I say that I have used polyhives 23 years. If I open the hive near freezing point, bees are not walking here and there. They are tighly in cluster.
Further more I install 15 W electrict heating after cleansing flight into hives.On those nights we have 10-15 degrees frost. In this case too hives have a tight cluster which tell that they protect their brood. If the hive is queenless, it is dispersed evenly in the box. Here you see a colony which had on 15W electrict heating in that empty part.
When a cluster enlargened in 10 minutes, it filled the whole 5 frames.Polyhive does not heat the colony. It gives only a shelter and reduce heat leak from the cluster. The cluster contol itself the proper heat.
Insulation saves energy and winterfood