Just jumped the 4 foot fence of the arena and escorted herself back to the barn. She will be the death of herself one day. At least since that fence is wood she could see it and didn't hurt herself.
Lesson learned: don't leave Gwen alone in the arena and walk back to my car when I already know she's worked up. Do A LOT of head lowering first and then go get what I need out of the car. Maybe. Better yet just take her with me.
Fortunately when she gets loose I know exactly where she's going to head- to Coriander's stall. I caught her back up, took her back out on the lead line and worked on head lowering, A LOT of it.
Ugh, I was hoping she'd never learn how to jump that fence...