What is this small metal box my hood fan vents through?

My hood fan vents into the top of this metal box, which then has a vent to the outside about halfway down the metal box. The box itself it maybe two half-height shoe boxes in size. I looked pretty hard, and the closest I could find was that it might be related to collecting condensation.

Does anyone know what this is for?

Thanks

I should say! The picture that comes up for this album is clearly the outside vent and is not what I'm talking about. Click on it to see all the pictures!