Hmm a lot of possibilities leh, you might wanna start by checking if the quality of glass is a good one. High quality glass like those use in automotives rarely behaves like that.
My shower glass looks clean and clear when/ after i cleaned it. But when there's steam and the glass fogs up, i start to see what seems like water stains on the foggy areas. It also appears if i wipe with a damp cloth. I have used various methods to clean the glass (vinegar, baking soda, dawn soap) but none seemed to work. Any idea why/ what is the issue and how i can solve it?