# Why is one face on my model gray instead of orange in edit mode?

I'm relatively new to Blender and mainly use it for viewing 3D geometry in various formats (such as OBJ). Currently I'm writing my own mesh voxelizer, and my program has support for outputting the resulting voxel array to an OBJ file containing a cube for each voxel. It's not incredibly effective, but I found it to be useful for quickly checking whether or not my voxelizer produces adequate results.

I have a three-dimensional voxel grid; every voxel that is a 1 (instead of 0) is represented as a simple cube, and all these cubes are stored in an OBJ file. The OBJ file contains only the vertex positions and face definitions, there's no MTL mentioned in the OBJ file (and no MTL file is generated, accordingly). Here's an excerpt from a typical OBJ generated by my program:

v -1,20000004768372 -1,20000004768372 -1,19999957084656
v -1,20000004768372 -1,20000004768372 -0,899999618530273
v -1,20000004768372 -0,900000095367432 -1,19999957084656
v -1,20000004768372 -0,900000095367432 -0,899999618530273
...
f 1 2 3
f 2 4 3
...


When I open it in Blender, I see that all geometry is correct, but one triangular face (always the first one in the list of faces!) is highlighted differently by Blender. Here's a screenshot (imported the OBJ, selected the whole mesh, didn't touch anything else):

(It's not a representation of any meaningful geometry yet, just a randomized voxel grid, for those wondering)

I'm not sure what Blender is trying to tell me here. How is this face any different from all the other ones (except for being the first face in the file)? What is wrong with the OBJ file my program generates? There's no color assigned to vertices or faces, there's no material specified, all vertices and faces should be the same.

Here's the file in question, if you want to examine it:

It's actually a re-created file so the random cubes might be different here and there, but the problematic face in question is still there: look for the corner vertex at (-1.2, -1.2, -1.2).