In my opinion, its the foundation settling somewhere. If you could find the center of your home and attach a water level along the radius of that room, you will surely find that the foundations is settling in the area of your hallway.
These cracks are almost impossible to fix. You are correct, they will expand in the summer and contract in the winter. It has to do more with water months than actual heat and coldness.
Without throwing more and more money on this area, I would just caulk as needed and live with it. Believe me, its there to stay.