"But that was not what Blomkvist had reacted to." Pg 324
In this point in the novel, Blomkvist finally makes a breakthrough with the case with Harriet Vagner. After looking at a series of photographs once more, he notices that Harriet is looking at something in the distance and her expression is completely different than in the other photos of her. Him noticing the expression change is what completely steered the investigation in the right direction from there on. If he didn't notice this small detail, the case might have been shut for good.