When Dory is in the back of Sarah's car on the way to the "frat party" you see her remove her hair barrette from the right side of her hair and place it on the back seat.
Then when they reach their destination she is again wearing the barrette in her hair.
Later, Sarah approaches Fallyn at school with the same barrette wrapped in a tissue and tells her that she found it in the back of her car.