Maybe I missed it, I haven't gone through this whole thread, or the other ones either.
Was it ever elaborated on regarding who warned Hank about the incoming assassination by the twins??
Is it a plot hole I missed or was I too drunk watching to show and missed who helped him?
You don't need to spoiler tag a Season 3 plot question in the Season 5 thread, by the way.
To answer it, though, the warning was made on Gus' orders, most likely from Mike. He wanted the Twins out of the way and he figured that Hank would be able to handle them, not knowing that he'd have been suspended and stripped of his gun at that point.
This was all laid out by Walt an episode or two later when he met with Gus in his office at Los Pollos.