Nononono, you misunderstand me. I'm talking about adding a NEW gun to a game. You are talking about using one already present in the game - the buses in Test Drive.
That's the thing though...How do YOU know that the gun is NEW? It could have been on the disc the whole time, but due to the recent bugs that have popped up in this game and the addition of these patches, something COULD have happened internally with the files. Perhaps it is a HUGE, internal glitch or it could be a mod (I still haven't seen enough proof to discourage the idea).
Trust me, man. I've been there! On my GTA IV, I used to be able to get police on me during Bomb Da Base II after killing all the guards and taking the Van. Ever since that one day that a modder visited, for some strange, internal reason, I can now rob the Van in peace while other players deal with the law. It is weird but I see it as less of a nuisance for me at least.
So, my point is, the gun COULD HAVE been already on the disc. Unless you work for UFG, how can you be so sure that the gun is NEW? Just because you saw it? I've never seen a Golden Meat Cleaver but I do know that it is still on the disc, waiting to be unlocked via DLC.