The note can be found in Garrahan Estate, on a table in front of the elevator on the first floor.
Look, I don't know where you are or what is taking you so long, but I can't get this sonnovabitch elevator open. I've tried the hammer, the drill, the blowtorch hell I even strapped some mines to the damn thing and barely a scratch! What the hell do they make these things out of???
I'm gonna try to see if I can find the keycard over in the Mining HQ. If not that, maybe one of those mining power armor things. If you get back before I do, hold tight.