Caliban said:You don't have facing in combat, since you are aware of danger and are assumed to be looking around constantly.
Outside of combat, nothing prevents you from sneaking up behind someone (although I would give them some heft spot bonuses as you approach if they are not within one move of your place of concealment).
LokiDR said:
There is no facing, period.
In or out of combat should not matter.
This change specifically disallows you sneaking up on a person in the middle of a large, well lit room.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.