Ok, here's my input: I have two Yorkies and they couldn't be more different than night and day. Brownie, who's my princess, will not walk on a leash, she hates walking and is carried everywhere because I guess I just spoiled her too much when she was a puppy. She doesn't like other dogs and will try to run away or hide from then, I swear she thinks she's human. We never had a problem with her weight and maybe she was a bit underweight before she got spayed and after that, she blew up like a balloon. She doesn't run around and play with toys, she loves to sit and watch TV, much like a stuffed Yorkie, you put her somewhere and she will stay till you come and pick her up again, that's why I'm allowed to take her to work with me everyday. Brownie also has a collapsed trachea and will snort and snort when she walks too far in this hot weather.
Pepper on the other hand, loves to play with toys, other dogs, people, you name it, it moves, it's his plaything. He's a good size now, but I bet that if he gets neutered, he'll gain weight as well. He loves walks and looks forward to going out everyday.
At 8 years old, this Yorkie you are sitting may have the same personality as my Bronwie, as well as some aliments due to her age, like arthritis or luxating patella (sp?). It's really hard to tell what they're like at such a short time that you've known him. As for the grooming, everyone has a different point of view of how clean their furkids are. Maybe call and ask if you can help them before the owners get back. Hope this helps you understand a little bit of unsocial Yorkies ~ like my Brownie.
Oh......but I forgot to say, she loves people though, because she thinks she's one of us, lol.