It's likely in the basement. There's a long hall going from the house to a set of freezers on the senate side and has lots of unidentified rooms along it. I saw senators going in and out if those doors and was told by a cop they were their secret offices. There's also a lot of dead space on the gallery level and I know for a fact that there's one door that led to the old national security cmt. In those days, we didn't even know who sat on that cmte.