Ok, so the assassin class article has made me aware of the apparent rule that any power you use, the target knows exactly what power was used on them and exactly what it does.
So if you are a hidden assassin (who didn't waste a feat) that places his shroud on a target, that target is immediately aware of the invisible shroud that will allow an assassin to do more damage to him if he attacks. Similiarly, a sniping ranger's target will immediately know he is a hunter's quarry, etc.
So.. my question is, how are monster's supposed to react to this? When I DM, I usually insist that players don't metagame, but at the same time, I also have to hold that up with my monsters. With things like defender marks, I understand that the power itself is designed so the monsters realize that they have been challenged by the paladin, or that the fighter is poised to strike them more readily if they dare focus their attention on someone else, and have the monsters act accordingly.
But for shrouds, quarries, etc, I'm not sure how monsters should react. Should I be running LFR mods like they are set in Order of the Stick (where everyone is aware of game mechanics in-character)? Orc Leader: "Oh snap guys! There's a heoric tier assassin lurking around who didn't take the Hidden Insight feat. He just put a shroud on me. Quick, everyone start moving around and making active perception checks to find him, while I move into a well-lit area."