Interesting. At first glance I thought "tape line" as well since I think a die break would certainly be jagged. That and the fact that you didn't mention that it was a raised or incuse line, but just a line.
Having said that, this is one of those images that plays tricks on your brain and your eyes. it seems as though below the line the cheek is a lighter color, making me think that something was covering it up. Ie, tape. However, I just so happened to have a screwdriver sitting next to me, and when I place the shaft over top of the line, the coloring is no different above and below the line. Plus, when doing this all of the "toning" seems to be more uniform than when looking at it with the line exposed to my eyes. It almost looks like something was lying loosely on top of it still allowing it to tone all over, but also giving a small line due to exposure to maybe sunlight for a spell, or just different lighting... I'm a bit perplexed.
But then again, maybe I am over analyzing this and someone will chime in with a "duh, it's simply "this""......