She answers by reaching over to take his hand, just briefly, squeezing it tight before letting go and returning her hand to the wheel. She's not sure what conclusion he'll draw from that fact. Will he keep her close as all he has? Realize that Gregson died when they were hundreds of miles away, so sending her away would not necessarily protect her? Or would he decide that he's dangerous, that he must push her away to save her?

The former makes a hell of a lot more sense to Joan. But she strongly suspects Sherlock will turn to the latter.