Cleaning Mirrors

I clean bathroom mirrors in public bathrooms and lately several of them have had a cloudy coating over the outside of the glass. The only way to get it off is to use a green scratch pad and scrub gently with glass cleaner till the pad starts to slide smoothly across the glass, then dry with paper towels. There's a reddish orange residue that is coming off which looks like rust. There is no smoking in the building of course so smoke residue was ruled out. Any ideas how the front of the glass seems to have a thin coating of rust on it? I just use plain old glass cleaner. I was thinking it might be because of the high humidity we've had lately, but we've had that many times before and never had this problem. The mirrors are about 30 years old and attached to the wall.

By Lori J.