I don't know about your dog specifically, but even well behaved dogs that mind the hens well can go after em.
My neighbors dog once got ahold of my girls during free range, I went out and saw only feathers, no chickens. I found 3 of the 4 around mine and other people's properties and the fourth was nowhere to be found. Several hours later I was returning from some errands, amd there she was; Hiding in the tree line and brambles that run along an adjacent neighbors property. A little shook up, but alive and well she was. And I have red sexlinks which are known for their vulnerability to predators of all types. Anyway, there's still hope that she will return home, if your dog was behind it. In some cases they don't return for a number of days!