I wouldn't even dream of ever leaving a single Weber grill outdoors, either covered or uncovered. Mine stay in the garage, covered always. The only Weber grill that I have uncovered inside my garage is my Ranch Kettle and that is because Weber still refuses to make a cover for it.
It is the only Weber product that Weber hasn't yet made a cover for. It is one of the most expensive products Weber makes and Weber hasn't made a cover for it. Due to it's size, it is one of the most likely Weber products that would be left outdoors and Weber still hasn't made a cover for it. It is one of the oldest Weber products dating back to the 1950's and Weber still hasn't made a cover for it. For years I have fought, argued, begged, pleaded, coerced, etc. Weber to please make a cover for this one lone, single product and they still haven't made a cover for it.