Katherine Solomon and Robert Langdon are dropped near Old Town Square by Sergeant Scott Kerble, who returns to the embassy to help Ambassador Nagel withstand CIA Director Judd’s pressure. Langdon stays evasive about his plan, arguing that if Kerble is questioned, he cannot reveal what he does not know.

Once they are alone, Katherine presses Langdon for answers. Langdon insists Sasha Vesna never fled Prague and is still in her building, despite Kerble’s claim that forensics searched her apartment. Langdon leads Katherine through the alleys to Sasha’s door, where Sasha’s Siamese cats watch from the window.

Langdon explains his realization about the messenger who slipped a note under Sasha’s door and then “vanished” instantly: the person likely hid in the vacant upstairs flat. He recalls that Gessner owns both apartments and used to live upstairs above her sick mother, meaning the building must have internal access to the second floor.

Inside the foyer, Langdon reveals a concealed passage beside a wall of stacked boxes that leads to a hidden staircase. He reasons that the note-leaver—likely Dmitri—could have slipped upstairs, then later entered Sasha’s apartment and moved Sasha upstairs (by coercion or incapacitation) so she would not witness Harris’s murder.

At the top landing, Langdon knocks and calls for Sasha, but hears nothing and finds the door locked. As a backup, Langdon uses the same key ring to open the upstairs apartment, deducing Gessner likely used one key for both flats and never changed the lock after Sasha moved in. When Katherine switches on the light, the vacant, shuttered flat is revealed to be filled with an eerie purple glow.