after rolling the front passengers window down and back up again a streak appeared horizontally across the center of the window. It looked to be oil and could be cleaned off easily with window cleaner, but would reappear as soon as the window was rolled down and back up again. I took the car back to the dealer many times and each time they found nothing wrong. They took the door apart and check the motor that automatically moves the window up and down. After about 4 years the annoying streak stopped appearing on the window. Always worried that something was wrong with the motor that moved the glass up and down, but it works fine. So no clue what the problem was.