There are many cases in the media these days where small dogs are stolen from the hands of their owners when they are out for a walk.
Some thieves bend down, pretend they want to talk to the dog, snap the leash and run off with the dog.
It is especially in the big cities where dog owners experience it.
One way to reduce the risk of theft of the dog, if it happens in this way, is to use a line with a carabiner that can be "locked" and thus make it more difficult to take it with you.
It can be liners like these from Ruffwear:
Ruffwear Knot-A leader line thick.