When this scene begins, Scottie and Judy seem content.
They discuss where to go out for dinner and Judy proposes Ernie's, the restaurant where Scottie first saw Madeleine and where he took Judy out for the first time.
Scottie agrees but is startled when he sees Judy, who's still getting ready, put on a necklace that he faintly recognizes.
Judy asks him to help her put on the necklace, and when he gets a closer look, he realizes that it's the one Carlotta wore in the portrait of her he's seen so many times while he was stalking Madeleine.
He knows. How can he confirm his suspicions, though?
Without revealing his plan at first, he decides to take Judy back to the Mission, the "scene of the crime."