What would cause soreness in the back of left leg above calf in the crease no other symptoms?

Muscle strain: is most likely. The worrisome condition would be Deep Vein Thrombosis (DVT). If you have a history of clotting issues, or recent long distance travel or surgery, you should see your doctor to get an ultrasound to rule it out. Hope this helps!

See below: The most common causes of pain in the back of the knee are a Baker's cyst, an injury to the meniscus, hamstring, popliteal tendon or ligament. Consider seeing your health care provider if it persists.

Tendon/baker's cyst: There are several things that can cause pain behind the knee. The most common are muscle or tendon strains or a Baker's cyst. The good news is that they are treated pretty much the same way with ice, rest, compression and elevation. If you have this pain with calf or ankle swelling you need US to rule out DVT but if it just behind the knee then the problem will usually resolve in time.