“You asked about the Fifth,” she said at last. “Why it’s the worst cohort. That actually started way before us.” She pointed to the back wall, where the legion’s standards were on display. “See the empty pole in the middle?”
“The eagle,” Percy said.
Hazel was stunned. “How’d you know?”
Percy shrugged. “Vitellius was talking about how the legion lost its eagle a long time ago—the first time, he said. He acted like it was a huge disgrace. I’m guessing that’s what’s missing. And from the way you and Reyna were talking earlier, I’m guessing your eagle got lost a second time, more recently, and it had something to do with the Fifth Cohort.”
Hazel made a mental note not to underestimate Percy again. When he’d first arrived, she’d thought he was a little goofy from the questions he’d asked—about the Feast of Tuna and all—but clearly he was smarter than he let on.
“You’re right,” she said. “That’s exactly what happened.”

The Son of Neptune, Chapter VIII