Chickens are opportunists and will, when given the opportunity, eat their own eggs or anyone else's. You didn't say if you also found some shell, which is the real giveaway that someone is an egg eater; one of my girls actually walked out of the coop with half a shell still stuck to her breast, brazen thing!
Egg eating needs to be discouraged as quickly as possible before everyone thinks it's a good idea. Many people, me included, use fake eggs in the nest boxes so that when they peck, there's no payoff.
There are also nest boxes designed so that the eggs roll backward and down so the hens can't reach them. I usually check nest boxes as often as possible, especially when everyone is laying -- and right now most of them are taking a winter break. Sometimes, they don't intentionally break them, but as many as three of my girls will all choose the same nest box at once and accidents are bound to happen then.
Good luck getting things under control. And, welcome to the community!
