I have three separate areas for my chickens. I have the main flock which is all large fowl and is in the biggest area. I have my bantam flock which is in one of the smaller areas and I have my grow out pen/breeding area. Before I had the grow out pen, I used to just have the bantam flock area and the large fowl area. I built the separation walls with future birds in mind and I didn't want my large barred rock rooster injuring one of my bantam hens with his love.
Anyways, I would feed all the birds in the two pens and every afternoon when I went to collect eggs, my leghorn pullet, Luna was ALWAYS in the bantam pen. This didn't make sense to me because there was one wall made out of plywood and another made out of chicken wire and a door for me to go in and out. The door was always shut and there weren't any ways for her to go over or under the wall. This happened every day and soon Luna was a big awkward giant in a flock of small bantams. I could never catch her doing it either. She would just always be in there somehow. One day, it happened when I turned my back for no more than 5 minutes! The next day I hid in the bushes next to the run, and spied on them. I watched luna eat her food, then proceed to jump from roost to roost until she got to the plywood wall of the bantam pen. She then walked across the side of my chain link wall of the run, where that horizontal retaining bar runs across the middle, and squeezed through the smallest gap. She hopped down using their roosts and started eating their food! I went in the run and removed her from the pen and she quickly ran and did it again. I sealed up the small hole and it hasn't happened since. I felt so silly knowing she figured this out and I for the life of me couldn't see how she was doing it.