I think I know how Demeter gets out of the pen now. I found her again on the top of the fence post, so she must be flying over the low end of the pen. Earlier I had discovered her out in the yard, so I went to get the camera. When I came back, she seemed to be telling the other chickens about her escape and they appeared to be listening to her. I like this picture of them talking through the fence.
She is easy to coax back through the gate, and there were no repercussions.