DracoSuave
First Post
I explain it in my head as the fighter parrying, blocking, attacking, feinting, etc. constantly against the marked enemy, therefore making it difficult for them to attack anyone else, and if they do, or if they move, they're giving the fighter an opening which he's especially trained to take advantage of.
This works with the whole idea of Wisdom (perception) being used as a bonus to opportunity attacks as well as the fact that the fighter can only mark someone he has just attacked.
So I would say that yes, the monster would realise they're being especially targeted by the fighter.
And if the Fighter wanted to be subtle about it... well that's what Bluff checks are for.
I like the idea of using skills to compliment your combat abilities in that way.