SOLVED - found the distant window it was attached to and deleted it.
I've got a weird floating sill, any ideas? I've restarted and can't select it. Also does not show up in 2D.
https://drive.google.com/file/d/1Yv2JNmZ_56u_HRI4L0pY-pEkl7LQMXq-/view?usp=sharing