Mist did that to me a few times, lol. It does look very strange but she did it a little different to most dogs that learn to do it. She looked really coy with her eyes half closed and blinking and would stretch the corners of her mouth right back and then lift her lips a little so it looked like a grimace rather than baring teeth.
One of her other daughters used to do it as well but I've not seen Jess do it.

It's so great when they smile back at you. No amount of money in the world could create something better.