Look...the car is almost 7 years old now, with over 100,000 miles on it. You can't expect Lexus to guarantee that nothing will happen to a car forever. Its an old car, and its a car that you don't really know the history of before the start of this year, you don't know if its ever been in an accident and repaired, and you live in a salty wintery climate. Lexus and the dealer will do, and should do nothing to help you. My guess is the car was repaired in this area at some point, that repair was of a substandard quality, and the rust is a result of that.
If you want a car that will be covered by a warranty...then buy a new one next time.