Might be obvious, but have you made sure you're slamming the seat home so it locks properly in the upright position?
I was being driven mad by an occaisional squeak coming from the rear. It would come and go, sometimes disappearing for weeks then coming back. Hunted for the problem for ages, took it into the dealer who tried to find a problem with the suspension.
In the end turns out it was the rear seat; sometimes, when I'd had the seats down (I regularly put a bike in the back) it hadn't clicked back home properly and would make this high-pitched squeak. Now I know to push it back firmly!