If you have a 2013 Z4 you could have a defective windshield. When I leased my 2013 Z4 35i there was a 6 inch dried water mark inside the glass almost directly in the center of steering wheel.This water mark shows up real clear when facing the sun, other times it's nearly invisible. Before I took delivery the dealer replaced the windshield with a new BMW windshield from parts.
Now again I have the same problem, same size water mark inside the glass in the same location, directly in the drivers vision.
My dealer in Scottsdale, Arizona says the BMW Rep has to see this before they will replace the windshield. It's now been a month and they have yet to arrange a service appointment with me and the Rep.
Check your windshields for this issue it appears that a batch of BMW Z4 windshields could have this issue.