Thank you for the advice! Yes, they have free choice oyster shell.
I had a thought though - nearly every day I had been taking treats out to them. Leftovers from our dinners, apples, bananas, carrots, greens, bread, etc. But the last 2 weeks have been incredibly crazy busy for us - as far as we can tell, this started happening just last weekend after 1 week of less treats & they've had even less treats this week - and it's happened again.
Do you think there's a connection? I'd hate to cull this hen as she lays nearly every day, but I can't stand wondering how many eggs she's eating & I certainly don't want her teaching the others to do it, too.
Thanks again!