The expression shifting darker in Regulus' look verified what Bastien had guessed at - he was either dangerous, or in danger, and either of those made Bastien's jaw twitch a bit. His sigh was almost imperceptible when he saw the other man reining himself in.
And there was a moment of obvious confusion at the question. "Mr. Karkaroff is the Headmaster of Durmstrang Academy, Sir. I don't see any reason why he would need be warned that there were others likely to see him on the grounds here."