You are waiting for a train sitting on a bench, say in Rome (it could be anywhere – Istanbul, Delhi, Kuala Lumpur, etc).
Somebody passes by and accidentally drops his wallet. You yell: “Sorry sir, you dropped your wallet”. He can’t hear you as some other trains might pass by.
As a good person you are, you jump off the bench and run 8-10 yards to catch up with him. He picks up the wallet, says thank you, and leaves in a hurry.
You are back at your bench 10 seconds later, but your purse/bags are gone. You’ve been a mark since the moment you sat down. They looked at the bag, calculated the moves, and knew exactly where to get you walking – enough for both of them to disappear at the perfect moment.
How to avoid: if you decide to go after the person, grab your bag, especially in busy places. However, now that you know somebody might drop a wallet in front of you in Rome while sitting down in a train station, you might even consider not to bother standing up.