You are going towards a security gate, get there and put your bag on the metal detector conveyor belt. Another person sneaks in front of you, seeming to be in a hurry. You are not in a rush, so you don’t mind. Then he gets held up at the detector, getting out several coins, his belt, shoes, etc.
While he is taking his time getting all the metals out, one at the time, the person in front of him (his accomplice, who went first to the line-up when he saw you approaching it) will just pick up your bag and leave, while your attention is caught by the guy “full of metal”.
How to avoid: well now you know it’s a possibility, you might not be so nice to the queue-jumpers!