Event: Cersei Lannister - Secret Discovery (Political Wins)

Episode 7: The Dragon and the Wolf at 62:50
Cersei figures that Daenerys has lost one of her dragons and has only two left.

Cersei: "How many dragons did you see at the pit?"
Jaime: "Two."
Cersei: "What happened to the third?
Jaime: "For all we know, it's guarding her fleet!"
Cersei: "She came here with her dragons and her Dothraki and her Unsullied. She came here to show us all her power. No, something happened. The dragons are vulnerable."
(Secret Discovery)