Did exactly the same thing, removed the 350z logo and snapped it in the process (understatement - they went really bendy and mis-shapen!). I also got a 350z shadow, I was thinking that the paints faded but its darker. I haven't had much of a go at removing it, just tried a bit of t-cut which made no difference. Keen to see if you have more luck! If not I'm thinking its a subtle 350z logo