Neither the entry on flanking from the SRD nor the Player's Handbook entry (p 153) nor the Rules Compendium (pp 56-57) mention anything about being able to see the flanker or being aware of the threat. However, this Rules of the Game Article, All About Sneak Attacks (part three) mentions
You get a flanking bonus from any ally your foe can see (and who is in the correct position to flank). If your foe can't see you, you don't provide a flanking bonus to any ally.
Is there a source for this or is this just an interpretation? (I am aware of the controversy around the RotG articles.)
I was reading Detecting a Held Charge Touch Attack which brought up the topic about how a spell caster threatens when holding the charge. It appears from most of the answers there's some question as to whether it can be detected at all, much less by other means. This then brings up the a related question, If your opponent can't perceive your threat, can you still provide flanking bonuses to allies in a flanking position against that opponent?
If flanking bonuses come from dividing the target's attention, it seems necessary for the target to be aware, but is this ever specifically mentioned in the rules?
It may be simpler to ignore this requirement mentioned in the article, but then it does create the odd situation that within the secondary reality of the game, something an opponent is not aware of has a significant impact on their defense.
I am interested in what the rules say, any indication of rules-as-intended, and recommendations on what works best at the game table.