Yeah, it's irritating when you don't see it until you've got it in Poser and are trying to work up materials.  Oh, there's more than one possibility for that artefact.  Could be that one poly has an opposed normal relative to the others.  Might have a duplicated vertex or edge.  Sometimes another poly occupying the same space sneaks in there.  May have one vertex that's not coplanar with the others.  If the poly is an n-gon (often go unnoticed when beveling) with a concave angle, that will screw it up. 

     Select that darkened poly of the door.  Is the number of vertices and edges what you'd expect?  If not, look closer. Try selecting all vertices for that door, note how many vertices are active, and then call the merge command;  If the vertex count drops, you've probably fixed the problem, but you'll also likely need to re-map that material zone.

     If that doesn't fix it, you might PM me and arrange to send me the troublesome part (door and the {air handling unit?} it adjoins).  Sometimes a second pair of eyes looking at the model afresh will spot something.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5