I have the exact same thing. It's not the hub - it's the motherboard. I have a Shuttle SB61G2 and called them up. "Why does your motherboard still supply power to the USB bus after the machine is turned off?", Answer: "It uses 5v and that's how we power the internal clock, etc. You'll see that many new motherboards do this". "Can I turn that off?" Answer: "No".
So far, the drain on the battery hasn't been too bad (no starting issues, the car is driven at least every other day, I've had the PC in the car for 2 weeks), but I imagine I'll pay the price in the long run.