Now she laughs. "You still do not know! Oh, this is rich. Well, you did not count did you? Four Doctors and only Three TARDISes? Do you honestly think the Doctor would ever be separated from his TARDIS if he had a choice?"

So, she's from the Doctor's personal future. Hardly matters, she figures that's why his box isn't here.

"Simple answer he is not. More accurately, I should have said 'Three Police Boxes.' There are four TARDISes."