When I first saw the Nokia 6 I asked myself "where is the antenna lines". I thought that Nokia was managed to release a phone without them, but looking carefully at some image at good lighting, in a specific angle you can clearly see them, like in this photo. Unlike some phone manufacturers that use different color, either brighter or darker for the antenna line bands, in the Nokia they lines are black. Because the phone is also black, it's very hard to distinguish between the rear plate and the antenna lines, it's like they are invisible and don't even exist.
There are antenna lines on both the top and bottom at the back side of the Nokia 6, and the lines are aligned with the to most and bottom most parts, which makes it even harder to notice them. I like how the antenna lines beautifully blend with the phone's color. I think for some phone the antenna lines actually fit quite well into their design, buy I think that for this sleek Nokia 6 model, a more solid look fits better because there is already the fine metal lines on the contour that gives the Nokia 6 it's distinctive prestigious look and I think it would be too much visual clatter if those antenna lines would have been more prominent.
In the press release image the antenna lines are more prominent for some reason, maybe because it's a rendered image and not realistic one. Here is a Nokia 6 Unboxing video, where you can get to see how the phone looks like from all direction and you can see how beautiful the black and the silver metal blends together, including the antenna lines at the back.
I don't know how it would look like if the Nokia 6 would be in a different color, like white or blue, whether it those lines would still be colored with the same color of the rest of the device. Not doubt that it's a beautifully designed Nokia phone!