What I believe to be the infill of cloud after the passage of a plane that created a distrail in cirrostratus.
An unusual distrail in low diffuse cloud appears as a line of holes. The planes passage through the cloud was from upper right to lower left. For more detals see the comments under the image in the comments.
Not a common sight. A plane flying for this length of time in a thin layer of cirrocumulus cloud is unusual if not unprecedented. The curved path is a bonus. The early part of the trail on the right in the image is already filling in with ice cloud. The plane at the time was still visible far to the left of the image.
The plane is in the image below.
Part of a long sweeping distrail in light cloud.