In fact, a sentence or so later, we are told that Christine's reaction suggests she is frightened: Quelque chose qu’on ne savait pas, assourdissait, alourdissait son chant… Il y avait, là-dessous, du tremblement et de la crainte.
I would assume she is afraid for Raoul, though it's conceivable that she is afraid of consequences for herself if the Voice notices him there. (This seems unlikely: the Voice has done nothing more at this point than go on annoying rants about her supposed feelings for Raoul, and she has demonstrated her lack of the latter sufficiently to merit the promised performance of "Lazarus", from which we can assume that Erik is happy with her treatment of Raoul at Perros-Guirec.)
In her letter of dismissal to Raoul she had begged him not to speak to her or visit her dressing-room again because both their lives may depend upon it: Surtout, ne pénétrez plus jamais dans ma loge. Il y va de ma vie. Il y va de la vôtre. (I suspect this may be an error of chronology by Leroux, since at this point Christine has yet to unmask Erik and she has no cause for physical fear of the Voice!) But in any case Raoul is not in her dressing-room. He is attending an opera performance as his brother's guest, which he has every right to do and which Christine can hardly be expected to prevent.
But it seems to me that the most likely cause of Christine's dismay and fear at seeing Raoul is that, as has been mentioned earlier in the scene, he looks much too ill to be in the theatre; she hasn't seen him since he came round in the inn at Perros, and both the managers and the Comte comment on his state of health ("Il a l'air malade", dans un état de santé alarmant). So the simplest answer is that she got Philippe's letter telling her that Raoul was ill and asking her what on earth was going on, didn't believe that Raoul could really be in any danger as a result of events at Perros-Guirec, and then realised to her horror when she caught sight of him that he really had been seriously unwell as a consequence of their little excursion.
At any rate that's the version I've used for my current chapter ;-)
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I would assume she is afraid for Raoul, though it's conceivable that she is afraid of consequences for herself if the Voice notices him there. (This seems unlikely: the Voice has done nothing more at this point than go on annoying rants about her supposed feelings for Raoul, and she has demonstrated her lack of the latter sufficiently to merit the promised performance of "Lazarus", from which we can assume that Erik is happy with her treatment of Raoul at Perros-Guirec.)
In her letter of dismissal to Raoul she had begged him not to speak to her or visit her dressing-room again because both their lives may depend upon it: Surtout, ne pénétrez plus jamais dans ma loge. Il y va de ma vie. Il y va de la vôtre. (I suspect this may be an error of chronology by Leroux, since at this point Christine has yet to unmask Erik and she has no cause for physical fear of the Voice!) But in any case Raoul is not in her dressing-room. He is attending an opera performance as his brother's guest, which he has every right to do and which Christine can hardly be expected to prevent.
But it seems to me that the most likely cause of Christine's dismay and fear at seeing Raoul is that, as has been mentioned earlier in the scene, he looks much too ill to be in the theatre; she hasn't seen him since he came round in the inn at Perros, and both the managers and the Comte comment on his state of health ("Il a l'air malade", dans un état de santé alarmant). So the simplest answer is that she got Philippe's letter telling her that Raoul was ill and asking her what on earth was going on, didn't believe that Raoul could really be in any danger as a result of events at Perros-Guirec, and then realised to her horror when she caught sight of him that he really had been seriously unwell as a consequence of their little excursion.
At any rate that's the version I've used for my current chapter ;-)