You have a fresh air duct installed. Not sure if the 'box' it is attached to has any type of damper and control. Some installers just put a small duct to maintain positive pressure on the hose, but you could have a motorized damper installed at the box w/ some sore of fresh air cycler (run time, humidity, temp). I would probably verify I needed MUA. Maybe a kitchen forum. Hood vents usually only run for a short period of time, and some homes are not all that tight to begin with. In extreme cases, your hood vent could draw through a chimney or if your water heater is in the closet, it could draw from the flue.