Thanks for your answers.
We caught her doing it yesterday and saw how she was doing it.
She is climbing up the welded wire with her feet. Not flying at all.
Its really peculiar, I have never heard of a chicken doing that. A monkey sure, but a chicken?
So I think I will need to put a "lip" all around the top of my enclosure.
Theres also a section where she could climb, and then drop down onto the roof of the coop, which would lead her to an easy way out of the pen.
I called my Mottled Houdan chicken Houdini, but it seems I should have given that name to Speckles (the Silver Speckled Hamburg) instead.
Wow! She was scaling the wall! I've never seen that before. So sneaky but I'm impressed!