When Evey wakes up in V's home, he tells her she has to stay with him because otherwise she could give away the location of his lair. She counters that she has no idea where they are, and he says the following:
You know it's underground. You know the color of the stone. That would be enough for a clever man.
This has always bothered me because it seems to be so insanely untrue. Those two facts couldn't possibly be enough to locate something. Could they?