The material below is Open Game Content from a 3PP Source
SWARM OF THIEVES (CR 2)
In crowded marketplaces it is not uncommon for a band of thieves to work together, descending on the area like a swarm of locusts. While this could be played out as a series of pickpocket attempts, the DM can elect to present the situation as a specific hazard to the marketplace; the DM should make a DC 20 Spot check for any characters in the affected area once for every minute they spend in the area in question, noting down the amount by which each roll failed on a scrap of paper (add these numbers each time in order to create a single, total value), being sure to keep the results for each player character separate.
The next time an affected character goes to check his coin purse he finds that he is missing a number of random coins equal to the total value (see above).
Characters that make a successful Spot check avoid losing any coins that round but unless the check succeeded by 20 or more (a result of 40 or greater) the area is too crowded for the character to have spotted the pickpocket attempt. The instant any thief is spotted the hazards breaks apart and ends immediately as the band scatters.