It's part of the first DLC Dead Money because there are Sierra Madre posters on the wall in there and that's the new location Dead Money will add.
Here is what The Vault wiki has to say about this bunker.
"The bunker was locked in patch 1.01. It is unknown if this was on purpose or a mistake. Given that there is no key in the G.E.C.K for PC and this location contains an important item, it is likely that it was a mistake. To access the bunker in an alternate way, simply open the console, click on the bunker entrance, and type "UnLock". This will unlock the sewer door and allow access.
It is possible to enter if the game cache is deleted in your console. The patch will be gone and Dean's Electronics can be acquired. Then simply reinstall the patch. (XBOX 360) Beware, after doing this and continuing with your post-patch game, you will lose the dialogue option to send your companions to the Lucky 38. Tested on 360 - saved before disabling patch and saved after looting Dean's Electronics and reinstalling the patch. Then talked to companion on first save, option was available, tried on second save (after reinstalling) and it was no longer available.
On PS3 it is possible to enter by deleting your game data via Game Data Utility and turning off Internet Connection in Settings. Get the Book. Turn Internet back on and restart the game to reinstall the patch.
There's probably a correlation between the "Sierra Madre" posters and graffiti and the upcoming Dead Money DLC, suggesting the possible presence of a Brotherhood of Steel chapter. Possibly Elijah,a former Brotherhood of Steel elder, left those marks. It was most likely locked in patch 1.01 so as not to give away anything about the upcoming DLC"