Ok so I think I found a glitch/way to get the Sentinel XS everytime. Ok, so go up to the store in West Vinewood, it will be around there the one that you can rob. Rob it then leave the store press start, and go to xbox marketplace. Load up the marketplace and then leave it. There should be a Sentinel XS on the street around you. Shoot the driver out, he will be going somewhat fast. Then you get your free car. I have done this and it spawns most of the time, if it didn't spawn the first time try it again. Can anyone help me test this to confirm it works for other people?