While last time I was in Barcelona, seeing group of young girls dressing in bikinis and casual clothing and looks like if they are going to beach at Barcelona's metro. They were the thieves walking around and getting in and out of metro trains to pickpocket. They just get in a metro train at one station and then leave the compartment at the next station and choose another compartment which is more crowded, and get into the train again and find chance to pickpockets. So be careful, tourists! (Because last time my tourmate nearly become the victim.)