Okay, so they've got to look for an orb that's missing from one of the tombs. That shouldn't be too hard, right?
Sophie anxiously starts comparing all the tombs, which have Knights sculpted in relief upon them. They're all posed differently, holding different things, wearing different clothes…but there's nothing consistent that makes one stand out.
Until she gets to the tenth Knight.
Instead of a knight lying in repose, the tenth tomb depicts a sealed stone casket. Even Teabing isn't sure why this one is different from the rest.
Their studies get rudely interrupted by the altar boy, who wants to know what's taking them so long.
In the process of lying to him, the altar boy reveals that the things they're looking at aren't tombs, they're effigies. (Tombs would contain actual bodies. These don't.)
The altar boy is surprised that "Mr. Wren" wasn't aware of that fact, seeing as it was his family who'd discovered the truth about the effigies. Ruh-roh.
Now the boy's super suspicious, but he goes to see if the door they heard slam was Father Knowles (the guy who Teabing lied about meeting them for their ritual).
Teabing's devastated. He was so sure this would be the place, but clearly it isn't.
Out in the annex, Rémy and Silas attack the altar boy, and tell him to run away as fast as he can.
He complies readily, thanks to the gun Rémy pointed at his head.