Kai is home.
Thank you to everyone for your support and help. Just a few hours after I arrived home to Chicago this morning, Andrea called with the good news that Kai had been found, thanks to a post on CraigsList and an animal-loving neighbor. Poor guy was lost in their neighborhood, but he made it through a tough Michigan snow storm. His paws are in bad shape, and he's a bit skittish and exhausted, but he's home.
Happy, happy ending. Now everyone go hug their pet. And if you don't have a pet, do yourself a favor and get one. They're the best.