Going... Nowhere?
You think you've finally managed to find the middle of the base, and you find yourself in a little cul-de-sac area that's pretty much empty aside from a very, very secure looking elevator door. You've seen some secure doors in your time, like that one in the back of the casino, or the one on the entrance to your hideout, but they look like hollow core compared to this thing.
Try as you might, you can't find a way to get it open. You even try all those little security key cards you've been picking up while you were here. Didn't I mention those? No? Well... you have been. They just weren't worth mentioning before now. And besides, they don't work here, so it doesn't matter. Looks like you need to go find the correct key card. Don't worry, I'll point out when you get it.