Ah Jon related this story to me as he just returned from his Europe tour. One of his group members were "robbed" in a train station.
As his group of four people just disembarked from a train in Paris, they noticed two men also hurriedly disembarked. Soon, one of the men knocked into one of the girls in the group who was carrying a bag like this on her back...
My analysis is that thieves always create distraction like knocking or stepping on your feet. When this happen, the companion will action at lightning speed. We habitually closes zip at the top and the thieves only need to pull the zip outward and the bag is wide opened. Closing the zip at the top is ideal to the thieves because the victim will not feel anything as the zip is being pulled open. Try it yourself.
So, when you go travelling, make sure that you either lock your back bag all the time, or carry it at the front or don't close the zip ending at the top.
What is your best suggestion?