(aka the black veil on unknown terrain tiles)
1) Activating "Smooth fog of war" let see one more tile through walls, sometimes revealing rooms behind those only 1 tile thick.
The smoothing should be done on the last revealed tile before fog, not the first fog tile after revealed.
(As an aside, seeing through walls could be a power)
2) There is no difference between "unknown" and "unlit".
Typically : You walk down an unlit corridor, and as soon as it leaves lines-of-sight it becomes black again as if never visited.
The walls hint there is a corridor, but :
- In case of adjacent corridors it becomes unclear what is "corridor" and what is "thick wall".
- It hides any know object.
3) Monsters that cast darkness effectively erase terrain knowledge.