Using "was" vs "were" for "as if"

"Were" is used for subjunctive mood: when the condition is contrary to reality. (And some other cases not relevant here; see the Wikipedia link at the end of this post.) Examples: if I were you (I'm not); wish you were here (you're not); if I were rich (I'm not); as if I were an unaffected spectator (I'm not).

But sometimes the condition is not known to be contrary to reality. "If the killer was hiding in the shadow, he could easily have come from behind": we don't know where he was, he may have been hiding in the shadow, so we say "was" not "is" simply because we're describing the past. "I don't know if she was aware of that. If she was, she should have said something": maybe she was, maybe she wasn't. So we say "was" not "were."

There's more on this from Grammar Girl, and from Wikipedia.