Langdon and Sophie enter Westminster Chapel, where Isaac Newton's buried.
They're both pretty apprehensive about how empty the place seems. They were hoping for more tourists to ensure their safety.
They decide to seek out a docent to help them find Newton's tomb, because there are gazillions of them, and it's a really easy place to get lost.
The Teacher is already at Newton's tomb, and he can't figure out the missing orb problem—especially because there's clearly a massive orb right on the pyramid that Newton's sculpture is leaning against. (See for yourself.)
The orb itself depicts stars, planets, comets…lots and lots of orbs. How is he supposed to know which one's missing?
As he's looking around, he notices Langdon and Sophie, and fingers the gun in his pocket.
He'd gotten through the metal detectors at the entrance by flashing his official rank. (Ooh, that's a hint about his identity. What kind of rank?)
He decides to pick their brains about the missing orb, so he comes up with a plan to lure them somewhere more…private.