Hello.
I was walking one girl home and it was particularly cold evening. Then I get home I took my phone out of pocket of my coat and found the same lines like on your device.
But when N9 warmed up a bit those lines gone and never came back.

Keep your N9 in a warm place, it's winter time

After the second time I sent it for repair, they couldn't find the problem but the lines didn't return when I got the N9 back. I also think it's the cold that was causing the lines.