I will give you an anecdote that happened to someone I know. Although Disney is a "closed off" parking lot unlike malls, there was a time a friend of mine went to Florida on a little shopping trip. Because it was such a short trip, they did not want to keep going back and forth to the hotel between malls, they decided to take all of their shopping (which was quite a bit) with them. After they were done at the second mall when they got back to the car to put away their shopping, they noticed everything that was inside the car was gone; it had been completely cleaned out. And further more, there was no sign of forced entry on the vehicle, which led them to believe that someone at the mall was probably scoping out rental cars from the company they rented from and had a master key to enter the car. And this wasn't a van or SUV either, so there wasn't really anything out in the open for passer-bys to notice and realize there most be a motherlode in there.
Around that time alot of Bahamians were being targeted in Florida though, so they may have been a target from the moment they left the rental office. I am only sharing this story to say that, I honestly wouldn't risk leaving my luggage in the car. I would rather feel more at ease knowing that my stuff is somewhere "safe", rather than constantly wondering or worrying about it.