He probably scoped them out at boarding time, looked for someone who looked loaded, made a mental note of her luggage she checked in, and then went to grab it at baggage claim.
That's true. He was not stealing Emma Jane's luggage from Jackson Mississippi. I didn't even think of that. He was targeting 'high value' luggage specifically. You are right. He must have known what he was taking.
Either he was surveilling the boarding line, or more likely, he had a paid guy(s) in luggage who were tipping him off. If I was doing this scam, I would put some kind of sticker (mickey mouse, cookie monster) on the case for him to know which one to take.