I travel a lot, most recently (week before last) to Barcelona. Anyone who has ever traveled there knows that the pickpocketers on Las Ramblas are crazy.
The problem with being pickpocketed isn't so much that you don't feel yourself being pickpocketed, it is actually getting the pickpocketer once it has happened.
Typically, a pickpocketer will bump into you, possibly even knock you over in the middle of a crowd. During the instant you are startled is when the actual pickpocketing actually takes place. And remember, the person who bumped you is not necessarily the one who pickpocketed you! As you are in a crowd, it is very difficult to determine who actually got your camera, wallet or pistol. And by the time you look around, get your bearing, etc., the pickpocketer may have already handed it off to another accomplice.
When I feel I'm in an area that is conducive to pickpocketers, I place my wallet in my front pocket. I move to the outside of large concentrations of people and I keep alert. If you look like an easy mark, you may be pickpocketed. If you appear to have a handle on your surroundings, the vermin will target easier prey.