I just assumed it was always un-vented, and just released air out the front through the grille. There is no vent pipe going up through the inside of my cabinet, the wall that the cabinet is attached to is an outside wall to the inside of the garage, so not sure how they would vent it, unless they went through the wall and up through the attic, which I doubt. Actually the retailer I bought this appliance from didn’t even install it, my electricians (who were also semi-friends) of mine actually did the install, so I highly doubt that they did some complicated install on this unit. I can only figure that they installed the unit and then saw that the filter needed to be in the grille and weren’t about to uninstall the thing to get this small filter in place, but they never passed on any update/info on to me so I was unaware. Heck, I didn’t even know it needed a filter, (except the grease filter I’ve cleaned underneath). So I think I’ll just have someone in place to hold the unit in case it comes disconnected from the cabinet when I unscrew the screw, hopefully I can drop the unit down far enough to get a filter in there.