Therapist is performing Thomas test for hip flexion contracture and found while doing the test PT was doing hip abduction and lateral tracking of patella, so which muscles you should stretch?

Thomas test primarily places the hip in flexion while testing for hip flexion contracture. Any deviant motion would be due to tightness of the muscle responsible for the motion and would therefore require stretching. In this case, hip abduction would be due to tightness of any (or all) of the following muscles; gluteus medius, gluteus minimus, piriformis, obturator internus and tensor fasciae latae.

The lateral tracking of the patella is caused by an tight/overactive vastus lateralis muscle which can be stretched. Tight vastus lateralis muscles are often accompanied by a weak vastus medialis muscle which in this case can be strengthened simultaneously.