Oh, and what about Dr. Whatsisname that so blithely informed Laura Collins how he was going to expose her. Right before he crashed and burned, by coincidence, of course. Maybe all four of our Doctors went to medical school together? ( Or was the other one not a medical doctor, we knew him for such a short time and my memory is so bad...)
Anyway, I rather imagine Dr. Lang's field of specialty lent itself to his moving around fairly often. For one thing, the graveyard in a small town can only yield so much before it is time to move on!