Over the course of a 6-second round, a character can look in just about any direction at some point, and can have looked in just about every direction over the course of the round. There is no non-arbitrary, non- suspension-of-disbelief-shattering way that facing can be implemented into most d20 sub-systems.
There are systems that can handle facing with varying degrees of success, and there are those that regularly do, but d20 is not typically among them.
There are systems that can handle facing with varying degrees of success, and there are those that regularly do, but d20 is not typically among them.