Yes, egg eaters are a big problem. I have had to go on egg eater patrol and even consider culling.

Now I leave random fake eggs so they peck something hard if they just decide to check. And I do collect often when I can.
I've also got the occasional brittle shell that a hen sat on and actually got stuck to her chest. Twice, 2 separate hens, not even in the same time frame. But you see lots of things in large flocks, that I would never have imagined.
But I had the same thing you describe... UNTIL I saw a hen run away with my own eyes.

She's lucky it was dark and I couldn't tell which hen!

On previous occasions though, I now realized THAT's what the nastiness covering some eggs a couple times in the past was.

And then one of my broody's discovered it somehow (maybe pushed another hen off the find) and was teaching babies!!

Glad that seems to have come to a stop. But I'm always on guard, especially when I have girls coming into lay that might be checking the boxes out frequently.