I think Watson is just a little unsure if this is a special kind of signal, he may feel a little strange in his heart, after all, he just talked to Sherlock about this thing, but at that time did not put too much on the heart, I think this is mainly a kind of ambush it, linked to the back of the single gang leader and the mysterious man (Moriarty) conversation, foreshadowing that things have not been completely finished