Joseph Buquet is a character the head scene shifter of the Paris Opera house. however, he liked to talk about The Phantom, whom he had seen. At the beginning of the book, he is found hanging in one of the cellars. They assume he committed suicide.
He has, but not alone. Near the end of the book it is made known that he stumbled into Erik's ingenious torture chamber, the entrance to which his body was hanging by. In his desperation to escape, he hung himself. To dispose of the body, Erik hung it outside of his lair. On greater thought, he went to retrieve the unique rope, which was made of catgut and known as the Punjab Lasso, which could have started a magistrate thinking.
Buquet is a perfect example in this book of someone being killed because he knew too much, or because he couldn't keep his nose where it belonged.