Okay! How old are your Olive Eggers?
You could try this method to see who's laying.->
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/
Are the three pictured the only ones that could have laid the egg?
I do think they're mixed breeds, where did you get them? Also how old are they?
Right now I'm thinking the culprit could be the first one. :D