Not sure it is still there, this was many years ago.
I read a couple of books and many articles that dealt with the NRO and Keyhole Series satellites. Those satellites are obsolete now, but I am sure hi-tech has expanded NRO's satellite capability greatly.
There's Falcon AFS (I think they call it Schriever these days)...but that's east of the Springs. And then there's Buckley AFB...but that's all the way up in Aurora. Do you think you might be thinking about one of those places? (They both have their share of "nondescript" buildings...and they both host NRO organizations)