It's not just on days you're not flying. For instance, in SFO you need to exit security and then go back through security to change concourses. Last week, I was on UA for SFO-SAN. I needed to get special permission to enter the AA gate area since it is a separate security area from the UA gates and my UA boarding pass didn't allow me to enter that gate area. Eventually someone from the AC came out with a permission form for me to enter the AA gate area.