It's genuinely rare to ask a question that Nill isn't expecting. Between trying to generally be prepared for most conversations, and her time as a telepath, most of the time Nill can tell where a conversation is going, even in just the general direction.

Somehow though Karkat manages to ask her a question that she not only didn't expect in the slightest, but that she has no idea how to answer. Normally she tries not to make it obvious if something like that happens - she tries to cover, to make it seem like there are a dozen options, but this time Nill can't help but to openly stare at Karkat, and to look, just for a moment, completely and utterly lost. So much so that she can't bring herself to hold his gaze for very long.

"...I don't know," She finally admits, because she can't think of anything better to say, and even if she could, a very small, exhausted part of her doesn't want to.

"Before now I always tried to stay with my friends or get back to them."

The implication there being, of course, that the preferred course of action is no longer an option.