Here's a notice from club member Mark Holzrichter:
Hey Colorado Model S owners, check the chrome between the tail lights. Mine is showing rust spots. It's just under 2.5 years old. Other chrome looks ok. Denver service center said it's due to magchloride on the roads and is getting me a replacement. I'm not going to bother replacing the chrome on my 17.5 year old Toyota which has similar rust spots.