I had 6 ducks go broody this year. Many of them lost their eggs one by one until there were none. I don't think it was a predator because many times I saw the hens eating the eggs. I think the eggs just weren't developing and they were cleaning out the nests. (I'm at high altitude and it's very hard to hatch eggs so I wasn't surprised they were having trouble.) Perhaps your hen got rid of the bad eggs from her nest.She had 6 now 4! I don’t know what happened.