One down six left

I checked on my broody hen today (day 17 of 21) and found an empty eggshell with just a little pink inside   
She still has six others to brood.  I looked around and could not see any signs of rats or squirrels getting in although the pop hole was open for her to stretch her legs.  The run is covered with weldmesh and there was no digging I could see.  I am hoping that this may have been one that was not viable and would not have hatched anyway.