In Twilight: New Moon, how do the Volturi eat the people who come on tour without being suspected?

In Twilight: New Moon, after Edward and Bella are dismissed from Aro's court, Demetri and Heidi talk about eating the people that are taking a tour of Aro's castle. How can they eat anybody who takes a tour of the castle without the people being reported missing and the Volturi being suspected of murder or vampiring, because everyone who enters their castle doesn't return?