This morning I found a half-eaten egg in the coop -- the second this week. It has been several days since anyone has found one to bring in. I even had to buy a carton at the store this week.
Egg eating is a very bad habit for a chicken to get into -- and difficult to break from what I've heard. We still don't know who the culprit is, so we're taking drastic measures to stop this before it is too late. I took out the nest box they've been using and moved the fake eggs (golf balls) back up into the top of the coop where they were laying eggs last winter. Maybe it will be out of sight, out of mind? And we're checking often for eggs to hopefully take them before they get eaten. Also for the past couple of days I've been putting out more "treats" -- produce, chicken scratch, leftovers.
If none of this helps, I guess we'll have to set up a chicken-cam or something.
Update: Moving the nest box up into the coop seems to have helped -- I haven't found any more broken egg shells.