"Dunno. Oh, I suppose he could be in hiding or in exile," he admitted, not sure if she'd sniff out an outright lie. "I was stripped of some key memories for a while, by the Time Lords, when I was much younger, for causing a little more trouble then they liked. Of course, they deigned to give them back to me after I saved them, despite that. Could be a more extreme version of that. Our people were reluctant to outright kill even their worst criminals."
There was no way in hell he was telling this woman about chameleon arcs and fob watches. The potential of the Master at his worst and this woman together - especially possibly without a Doctor to put the breaks on - was an unpleasant thought.