Why are some of the faces on my mesh lit up? I'm relative new to blender, and I found myself working on some models for a game.

Recently I was modeling this door, but at some point I've noticed that some of the faces are lit up for some reason.

I tried some things I knew about but none of them worked yet.
Maybe someone knows how to fix this problem



I hope this image provides more info.
I've also tried to recalculate and flip the directions of the normals, but none of these are fixing it.
If I smooth the mesh it looks way more weirder than having it on flat shading.

The mesh keeps its lit faces inside the game engine too.

I'd reconsider making a totally new mesh, but I'm still curious about the faces acting this  way.
First off, you should check where your light is coming from.

If the light is close to the door, the dents of the doorframe might block light, only parts that are raised a bit will catch the light. You could easily check that by placing another light source in the scene. 

Another thing you should check is if your normals are ok. You can tell blender to recalculate these when you go into Edit Mode, select the affected faces of the model (should be ok if you just select the entire model) and then press "Recalculate" in the left tab under "Shading/UVs". Flipped Normals can often cause lighting issues in the viewport as well.