There are several shells hidden in the code, you can see them all at http://www.destinydb.com/search?q=shell but currently there's no way to access them. I know that some people glitched into a HoW area and they found a new shell on a dead ghost, but those who used it were supposedly banned. There's another Shell that came with special editions called "Frontier Shell", you can see it on any of my characters. EDIT: Added picture because I have nothing to do right now...

There's a preorder only shell that some players have. It's red and I think called Enforcer. You didn't just preorder but I think it was a specific version of the game that when preordered came with the download code. If you look online there's multiple shells depicted on some sites but the only reported in-game sighted ones are the default/general shell, Enforcer, and Intrusion. Intrusion is supposed to come with the HoW dlc but some supposedly have it by glitching into a unreleased area or by some form of hacking. I'm not certain how reliable my memory or sources are, so don't take my word for it.