Actually, it was user error. I parked one wheel on a tallish dirt mound, and that created enough extra space underneath the tractor for four criminals to make their break. Two respectable girls stayed inside where they belonged.
It was a great learning experience for all of us. The chickens didn’t disappear or run out to the road. They just sort of followed me around looking really self-satisfied and investigating the wide world as I wheeled the tractor and the two good girls back toward the pen. I let them enjoy their freedom for a while, until they started heading down the woods path with dusk moving in.
I called the Missus and she helped me gently steer them toward their open pen, and in they went.
I did like seeing them wandering free in the yard, but that won’t happen permanently. There are just too many bad things in the woods. It was fun while it lasted.