If there's going to be a day1 patch, they'll most likely be in there.
Obvious signs of this being the case will be a patch size above 100 megabytes, with little to no new music or video content. Code itself doesn't take up a whole lot of space, but 3d models and textures do.
It's understandable that they wouldn't get everything on the disc for the day they go gold (they have to be prepared to have 5 million units in stores on release day, so work backwards), but if the art department was halfway through something or was just starting something then, it should be ready about a week before day1 (when they need to start organizing the patch contents).
The unlock codes are there because the content gets patched in regardless of whether you buy the DLC or not, so you can see others online even if you haven't got it.
EDIT: I'll make it clear I'm not condoning day1 DLC, but this is a type of game that can a team can continue to work on indefinitely, so to a degree the release day is irrelevant to a fair chunk of workers there, because after the game ships they're still going to be continuing to work on it.