It looks like some kind of injury, maybe internal injury, like those bloody eggs you see turn up in here once in a while, maybe the egg broke inside and was being passed and you know how gross chickes can sometimes be, they will eat that shell..so no trace of that ..its interesting..hope it clears up..I would note who is roosting right at that spot if you can..When things happen here, I have been out at night with my flashlight to see whos here or there and to catch hens for the breeding coops ect...might find the culprit..
