It's hard to say. It could be a snake, or it could be that something is able to pull them through the netting. Hardware cloth, 1/4" is a better preventative. At that age, the chicks are still small enough that a snake might be able to eat them and get out through the netting. Good luck, I hope you're able to save the last chick. Whatever has been feeding at your pen knows there is an easy meal to be had.