The only other thing I can think of it the egg has traveled back up the oviduct (reverse peristalsis of the oviduct). Hopefully it has not dropped into the abdomen.
You mention it's time to worm your chickens, do you use Metronizadole to worm your flock?
I was not aware the egg could travel back up the oviduct
Learning never ends & thankful for BYC Members knowledge & experience 

