Pain over buttock area radiating to front and lateral side of thigh. Does not go beyond knee. Pain increases with hip IR in neutral or 90deg Hip flexion. Some stiffness felt. Pain aggravate when sitting and walking.. You treated piriformis. I presumed hip flexion / adduction maneuvers because this was also positive with pain.

I would put the hip joint as high up in my hypothesis list.

I would confirm with additional tests such as hip extension and abduction ROM (Active and Passive). If hip, these ROM would be painful as well (less so than IR).