Tony's ring was an engagement ring that Tony bought for Sally, in order to propose to her. He stored the ring in his car. One night, Lee stole Tony's car, found the ring in it and proposed to Kelly while she was drunk. Later, they cancelled the wedding, but Kelly kept the ring.
While searching for clues about Tony's disappearance, Sally found the ring in Kelly's locker, but was unaware of the fact that this is actually her ring and returned it to the locker.