Hi David,
I have had a look for a few examples of dodgy subscaps,
I think ultrasound is not as sensitive as MRI for subscapularis injuries, especially in the superior part of the insertion. When I see a long head of biceps tendon which is drifting medially I suspect a subscap tear causing long head of biceps instability. Medial migration of the LHBT may also be due to a supraspinatus tear commonly.