[John does decide, while he gains on her, that Sherlock ought to have a reason for it, and considering the pace she's moving in he had a reason for not explaining too even if he would surely have enjoyed saying how he deduced it. And it hits him that it's probably best to be as casual as possible, and so not saying anything might be a little... odd.

Maybe it's a little late to realise it, but. Ah well.]

E-excuse me, miss.