The same thing happened a couple of months ago. My husband who is a professor at UBA witnessed it. There was a big conference, and after his talk one of the main speakers (famous guy) stood around chatting with people, with his backpack at his feet. In his backpack: his laptop, his keys, his money, and his agenda (containing his address). When he bent down to pick up his backpack so he could leave he realized it wasn't his backpack any more. Some one had switched it for a similar one. In it was a coffee table book toughly the same weight and shape as his laptop. When he got home the thief had already been to his house and robbed it...... People at UBA said it happens ALL THE TIME, especially during big conferences.