Clarisse and her cabin attacks Percy when he's alone.
After the flag is captured, a hell-hound appears.
After the hellhound is defeated, Percy sees the symbol of Poseidon floating over his head.
Uh-oh... this is not good.
"They surrounded me, and Clarisse thrust at me with her spear. My shield deflected the point, but I felt a painful tingling all over my body. My hair stood on end. My shield arm went numb, and the air burned. Electricity. Her stupid spear was electric. I fell back." (p. 123)
"Di immortales!" Annabeth said. "That's a hellhound from the Fields of Punishment. They don't ... they're not supposed to ..." (p. 125)
"By the time I looked up, the sign was already fading, but I could still make out the hologram of greenlight, spinning and gleaming. A three-tipped spear: a trident." (p. 126)