Mr Blumenstein:
In The Seven Crystal Balls at the theatre Haddock remarks that Bianca follows them around everywhere, when he hasn't seen her at all previously.
I think I misread this while posting earlier. Given that it was Haddock's comment on Castafiore at the
Crystal Balls, the reason would have been the other way round.
Since the Captain was a regular at the hippodrome, Castafiore's songs would have kept ringing in his ears, haunting him.
He was very interested in performances like the water-to-whisky magic but the Castafiore part was one annoyance during his visits to the shows.
There is another instance which indicates Tintin had met Castafiore for the first time in
Ottokar's. In
The Castafiore Emerald wherein upon her arrival at Marlinspike, Castafiore asks Tintin whether he can recall their first meeting in Syldavia.