"They wouldn't drive sheep by themselves, surely ?" "Oh, of course, he started them.

Once he got the sheep out of the paddock, he started the dogs for home, and rode off.

You see his plan.
If anyone finds the dogs with them, of course he had nothing to do with it.

Sheep-dogs will often go into a paddock, and bring a mob of sheep up to the yard on their own account.

It's an instinct with them.