Okay, I can see the bottom now...there's not much space down there, but on the wall in front of the bottom of the staircase, there's a pair of large doors.
They sure put a lot of work into this, really emphasizes the 'hide' in hideout. With the space that the staircase is in being so narrow, it'd be easy to miss in the blueprints. And if this is really the only entrance to this hideout, it's no wonder they've managed to stay undetected.
Still though, they're gonna regret only having one exit...because with this, they'll be trapped with me. I almost pity them...but that won't stop me from massacring them.
Fortunately, there's no cameras down here, none that I've seen, anyway. Still though, they've probably noticed that all the cameras on the top floor are offline, and there's also the fact that they can't communicate with the guards I killed...yeah, it's likely that they're laying in wait for me.