[Stiglr]

When I see these, I usually find that they're polys that have been "turned inside out" by the positions of the vertices relative to each other. Say you've got a flat surface to that box and the vertices are misplaced (usually by drag and drop moving while you edit) such that there's an unnatural "dent" or contour in what should be a flat surface. That would produce the "red line".

Try moving vertices individually until the line disappears (it will do so instantly as you move vertices, once you solve the problem). Or, try subdividing the surface; new line edges between vertice pairs might also solve the issue, or make clearer where you're creating problems with the shape.