Imaro
Legend
http://www.youtube.com/watch?v=rS5tzGfOBGU MAGIC!
But seriously, do you imagine the player using this power just says. "I just stand there and I'm invisible, not even trying to hide."? If so, then I imagine your confusion. Personally if this power was used I'd either ask the player why he is invisible or just make up why the enemies dont see him.
Perhaps he is posing next to a statue? Or dropped to the ground just in time? It's all dependant on the situation.
Ok, but do you also see that if you say he's using some form of non-traditional magic... it irons out those weird situations?