I actually thought about this same thing, but as someone who owns one, unless you have it thrown open there is no way. First of all the sides are closed underneath your arms so it would be impossible for them to slip the whole thing out...in addition, if you have the snap closed its larger than the opening on either side so they would have to grab, manipulate, and pull like crazy to get it out... imo pick pockets don't want to be caught, generally when you are pick pocketed you don't realize it until you go to use the item that was stolen... this really makes it nearly impossible... plus you still maintain the benefits of things not being able to fall out..
