By chance do his legs look like this:
http://www.yorkietalk.com/gallery/data/500/224dec.jpg
Up close:
http://www.yorkietalk.com/gallery/data/500/224hair.jpg
This is my dog back in Dec. I went through the same thing. I didn't really notice he was "missing" hair until others mentioned it. It appeared as if someone had buzzed his hind legs when in fact no one had touched him.
We never took him to the vet b/c he showed no distress and had no change in personality or activity. In the past month or so I've noticed the hair is definitely growing, so Im not too concerned anymore.
Here he is a few weeks ago; I shaved him so that his hair matched his legs and he looks great now!!
http://www.yorkietalk.com/gallery/da...224pic0033.JPG
I hope this sheds some light on your situation, but no matter what keep an eye on your dog to make sure there is nothing seriously wrong. Good Luck!!