Since they never use Box 3 I am thinking the hen laid it while on the jump bar which is right outside Box 3. The egg didn’t break since it hit onto a pile of straw. That is my guess right now. I have a jump bar a foot off the floor, then the roosts…one at 2 1/2 feet off the floor and the other about 3 feet off the floor. I kept the jump bar in there since they seem to prefer using it to help get up there. When they were younger pullets and first introduced to the coop, they never at all used the roosts and I figured putting a bar in there a foot off the floor would encourage them to roost since in the brooder they also had a bar to teach them roosting, which they did use and play on. But, my chickens are so dumb it took a racoon killing one of them before they learned they needed to sleep up high on the roosts.
It’s about time for me to check the boxes again and hope for an egg but it’s raining right now.