This is Missy. The day we brought her home from the shelter, she jumped out of the truck and took off. She eventually was found and brought back. Has your dog ever just took off even though you told them to stay?
There are ways to train your dog not to dart out!
Every time you or your dog are going through a door/doorway/gate etc. make him sit. Then open the door or proceed through the doorway, but keep him in the sit position. If he stands up, close the door and make him sit again. Then do it again. Do it every single time.