They hover only in the centre of rooms, with a very specific behaviour: they go one direction for 20-30 cm and then suddenly jerk back toward the centre of the room. They are always in a cluster, but I don't think they are cluster flies because they don't make any noise. They don't seem to be mind being misted with water, or smoky rooms. They also don't seem to be particularly attracted to light.
They are silent, don't land on food and don't seem to be causing any problems except being distracting and in the way. But that's enough! Does anyone know what they are, or how to get rid of them?