I also found a very nice car that's hidden. You can hear nitrous as you shift gears, and occasionally fire will shoot out of the exhaust. You must have the last island unlocked, Alderney I think it is. Go to the map, and zoom in on the top right hand corner. You will see a road that branches into 3; take the middle one as it ends with a large, run down house. Next to the house is a garage, go behind it and sort of hidden in the bushes is a Sultan RS.
That's the Infernus. I found it once in XBox Live.