I too have gotten an egg encased in some kind of membrane. I have no idea which hen was the culprit. I currently have 10 , 7 month old, Barred Rocks, New Hampshire Reds and get 5-7 eggs per day.
I too, had a 7 month old hen lay the same thing. I almost looked like the duct came out with the egg. I have no idea which one did this but they are all still alive.