I had left the barn door open for a few hours that day and the light was off. The chickens love exploring the barn and spent time in there. So maybe the chickens just decided they were too lazy to walk back to the coop. But then yesterday I found another egg on the plywood and the barn door had been closed all day. And tonight another egg on top of an opened hay bale.
There are many gaps around the bottom of the barn. It's enough space for our cats to come and go, usually while chasing a mouse. So it makes sense that a chicken could fit under too. I'm not sure why the barn is preferrable to the coop. At least five of the eight are laying in the coop. I'm not even sure who the rouge chicken is.
What to do? I don't want to have to hunt in the barn every evening for possible eggs. If we miss a few, it will just attract rats and I HATE rats. We have decided to keep the chickens in their run for the next three days. The run is plenty big enough for all eight, but it is void of vegetation, just dirt, and the chickens will be unhappy about not having access to the orchard and its luscious grasses. Hopefully three days will be enough to break the habit and get all laying back in the coop. If not, I'll be looking for suggestions about how to train a chicken.