where had katniss seen the red-hairedAvox before

I believe Katniss had seen her in the woods at the beginning of the story when they saw the hovercraft.

Katniss saw the red-headed Avox girl in the beginning of the novel in the woods. She and boy were caught by a hovercraft. The Capitol speared the boy, killing him, and then they cut off the girl's tongue as a sign of guilt of a crime.