Your poodle is licking that spot. Doggie saliva stains coats. On white dogs it turns it a reddish brown. She was ichy right in that spot is why.
Currently, I am a dog grooming student. You see this alot. Nothing to worry about, but check for fleas.
Could this be something that has bleached her fur? Something which has got on her? Does she go anywhere there are chemicals or anything which could act as a bleach? In the kitchen or garage? or somewhere else?