I know that it just sounds so despicable that it is hard to believe, but people steal dogs every day. Your dog can be sitting on the front porch and someone will stop their car, walk up, take him, and drive away. You may have left your dog in the back yard while you went to your neighbor and someone walking by could open the gate, click on a leash, and whisk him away.
Why do they do this? More than just being bad, bad, people; they normally do this because they want to sell them. Many people “fish our neighborhoods” with orders for dogs. Once they find a dog that someone wants to buy, they simply scoop it up and away they go.