This was definitely a moment when Marguerite was relieved that she wasn't infected with the singing thing and had only been responding as such when she either felt like it to make someone not feel so singled out, or because it seemed necessary to get some sort of point across and there weren't actual questions being asked so she.... just had to respond. Which meant that she didn't have to worry about singing at an inopportune time.
"He wanted me to learn the identity of the Scarlet Pimpernel as he knew you'd be at the Ball. I told him he was mad, and then he told me that if I did not, Armand would be guillotined because they had learned he was one of your men."
That was the crux of it. Why she hadn't told him? Well, between Chauvelin being right there even as she said she didn't know what to do and Chauvelin taunting her with quick decisions are sometimes best and the fact that the gulf between her and Percy was so wide at that time that she didn't feel she could tell him even if she'd been alone with him.