What did Percy notice about the new sword instructor, Quintus?

(Battle of the Labyrinth)

Percy noticed a strange mark on his shoulder i think. At first he thought it was a tattoo, but them realized that it was actually a brand. The mark was that of a bird, he was branded by the goddess Athena when he accidently killed his nephew.

He seemed odd. he also found it strange on how he was seen around the entrance of the Labyrinth. I just read this