In 08, every Chevy was fitted with On-Star, correct?
Now that it's a missing persons case with the legal authority to do it, without knowing the exact capabilities of that system, I'd think that particular vehicle would be located pretty quickly...
There are some good folk, most of them bulldogs, that run the DOT in the delta. Give as much info as you can on here and I promise it gets read by the right people.
I'm in the area a fair share too. Let me know what else I can be on the lookout for.