Sun glint.
Like all spiderwebs, you can only see part of it when the light hits it from a certain angle, and I can't move around to different sides easily. What's in the photo is what I could see. So I'm not sure how much further up it goes, or whether or not there's a crosspiece above as Alcaray suggests.
The orb spiders here can run lines across remarkable distances between two objects, particularly ones I walk between in the morning. I'm not sure how they do it, although there are several plausible methods