I think you just missed the egg the day before. Could have been shoved into a corner of the box and covered by hay. Had that happen a few times, myself.
I actually had an Easter Egger who gave me 2 perfect eggs in one day. Yep, no mistake....she did indeed. I was out in the coop early in the morning and kinda smiled as I saw Agatha on the nest - she hadn't been laying very long, maybe a couple of weeks or so. She stood up and dropped a nice egg, and I reached under her and pulled it out, still wet.
Later that night I went out to lock up and do a head count. (am I baaad chicken mama because I don't worry about that kind of thing anymore?) Aggie was back on the nest instead of on the roost where she was supposed to be. Just as I went to shoo her out, she stood up and deposited another egg. I grabbed it, expecting some kind of flaw since I'd never heard of a pullet laying two a day before, and again it was still wet from being freshly laid. AND there is no mistake about identity of the chicken...she was the only blonde Easter Egger I have.
So while it's more likely to be an egg that got lost in the shuffle and not found until later, I will be the first one to say that it could be a similar situation to what Aggie did. None of my chickens had ever done that before, and none have ever done it again, but it happened.