This one has got me really stumped! I'm thinking it has to be somewhere in Arizona, like secluded mountains in the Lake Havasu region.
It just cannot be in NCR territory because the NCR govt. would certainly have found it before Elijah and Dog would have had to drag you through the Mojave outpost to get to NCR territory so I think we can safely rule out California.
The starting point of the DLC is the Abandoned BoS Bunker which is located on the east side of the mojave map on the edge of the Colorado which is the border of Legion territory so it's gotta be to the east or northeast which puts it squarely in Legion turf but how come neither Caesar or his Legion seems to know anything about it or have tried to get into it?
Maybe since Caesar and the Legion are fundamentally against advanced Pre-War technology that they just left it alone but Ulysses for sure has seen it and knows its location as he is the one who first told Elijah about it?
What do you guys think??