[ Apart from the very obvious given his features, there's a unique aspect to his newfound friend's nature that intrigues the Doctor on a deeper level than he would betray on the surface. Friend, of course, because the other hasn't completely run away from him yet, thus a positive reaction, thus auto-friend until or unless told otherwise.

The Doctor is used to being a few steps ahead and playing oblivious. But, despite enjoying the chance to make it known how very clever he is whenever he gets the opportunity, he's also the first to admit when he's wrong. Perhaps he's wrong about his suspicions now, in fact. More to the point, he finds no reason to mention it. Yet.

A coy, slightly mischievous, slightly knowing and most assuredly annoying little smirk spreads on the Doctor's face as a reasonable guess is offered. ]

How did you know?

[ He likes to deflect. Prefers it. Particularly when talking about his true nature, his past. But he can give a little, he supposes. A true thing that still doesn't betray much about him. ]

A fool who wanders about across the stars, observes time and space, straightens out the wibbly wobbly bits. Ever in search of the most perfect cup of coffee, yet to be bested by Elisabeth Pepys.