So the snake is ok after the shed and everything is normal now around the vent? Was the shed thicker around that area then that makes you think it's retained shed that has come off this time?
I know it costs money to take snakey to a vet, but if you're still not sure, you really need to be a responsible owner and get it checked, whether snakey was cheap or expensive; you wouldn't want your pet to suffer any unnecessary pain or suffering