Bertrand left the dressing room, fully changed back to his normal clothes, expecting to find Beatrice in their meeting place. She wasn't there. That should have been his first clue that something was wrong.
A crew member told him she had seen Beatrice leaving backstage. He guessed she probably went to greet her fiancée. With that in mind, he went to Esmé's box, where all of them should be.
What he found outside was a commotion, with regular audience members, police officers, and official firefighters crowding the entrance of the boxes. But only when he heard the sirens outside he realized something was terribly, terribly wrong.
Part of him wanted to find his friends, but another knew it was best to leave and not get involved in whatever that was. Soon there could be journalists, and being seen in something like that was not good for him and his family and their organization.
Bertrand turned around to leave, when a small object forgotten on the floor caught his eyes. He had seen one identical to that not too long ago. Small, easy to hide, but very deadly. That one couldn't have hit anyone, but there must have been others. Beatrice had a perfect aim.
At least the victim would have died quickly.
Making sure no one was looking, he picked the dart up, so there would be no evidence. A competent investigator may have been able to link it to someone from the organization, either by identifying the poison on its tip, or the bird from which the small feathers were taken.
He left the theater cursing Beatrice and her impulsiveness. It seemed like it was a bad idea to let her keep the darts. Now the plan was ruined, and who knew how long it would take to clean up this mess.