How is Count Fosco exposed to the Brotherhood, an exposure that leads to his assassination in the novel, The Woman in White?

At the opera house Walter takes Professor Pesca to the showing of Lucrecia Borgia, where he points out Count Fosco. Pesca does not recognize Fosco, but Fosco recognizes Pesca and flees. Pesca and Fosco belong to a brotherhood of political assassins and Fosco has betrayed the brotherhood, a deed punishable by assassination by any other member who finds him. A third assassin, the slim man, witnesses Fosco's reaction to Pesca and follows Fosco until he kills him in Paris weeks later.