I had something similar happen a couple of weeks ago. After driving a few miles with the top down, at a light I smelled burning brakes. Looking around, there was a truck next to me so I just assumed that it was the culprit. But just in case, I made sure the parking brake was not on and it was, in fact, down. After pulling into the driveway, the smell was still there. It was the passenger side rear brake that had overheated. I took it to the dealer the next day but the problem had disapeared by that point.
I still dont know what happened. I have noticed that after parking out in the rain ( I dont have a garage
), the brakes stick a bit when I back up in the morning from what I think is a bit of rust. My current theory is that there was some rust that caused the brake to not fully release and that is why it over heated. If anyone has a better theory, I would love to hear it.