The hens are pretty good at cleaning up any broken shell. Do not expect to see any shell laying around. Sometimes you are lucky to see the remains of the egg whites.
My first thoughts are that you probably have an eggeater. Whether the hen is cracking the egg intensionally or the egg is cracked when laid and she is just taking advantage of the opportunity, I can't say, but you need to get a handle on it because it is a learned and taught to the others behavior. If she is not cracking the egg on purpose now, she soon will be.
The other possibility that comes to mind is that you have a rat eating the egg and the hens are eating the egg shell.
Good luck!
