Do you have a light tint? Maybe it has cracked/crazed.
Are you sure it's inside? Maybe it is scratches in the glass.
Can you feel the marks? Hag you tried wiping them? Windex?
If it is inside and yo can feel it but you can't wipe it off with a mild cleaner, do NOT try and scrape it with anything. You will damage the defrost elements if you do