She pleads ignorance!

"I had no way of knowing where you were keeping Monica," Edelgard replied, her voice distorted by the mask, but conveying a calm she didn't feel. "It's not my fault the bandits we both utilized fled to the fortress in question." This was of course, a lie. Hubert had discovered Monica's location through some sleuthing. The bandits, which she had claimed were supposed to kill Claude and Dimitri, was in fact a gamble to weaken the Church with a few casualties, as well as remove some pawns Those Who Slither employed. Two birds, one stone.

"You... Didn't know? That's your excuse?" Arundel asked, taken aback by the excuse. His expression then turned thunderous. "You lie. You knew the bandits fled to one of our hideouts."

"I had told Kostas to flee to the Red Canyon afterwards," Edelgard said smoothly. "But he was killed by one of the mercenaries who intervened. And apparently, he failed to deliver that plan to his next in line in the chain of command. I did not expect them to flee to a shared hideout against my orders. And as I said, I had no idea Monica was in said hideout. You never told me where you were keeping her. Furthermore, the Professor of the Black Eagles, and by extension, the students, including myself, were only informed of the details of the mission the Church gave me shortly before we left. I did not deem it worth the risk to potentially expose myself by informing you we were coming on short notice."

"And because you didn't warn us, Kronya is injured, and many of our own number dead," Arundel replied.

Edelgard shrugged. "I assumed that you would all just flee and leave us to kill the bandits. A few loose ends cleaned up, no harm to us. As I said, Monica was unexpected. And I never attempted to kill Kronya, only look like I was trying. If I truly wanted her dead... She would be," Edelgard asserted. Another lie. She HAD tried to kill Kronya. That said, she wasn't above using the opportunity to claim otherwise that her survival offered.

"And as I said, the Church were the ones who ordered the mission in the first place. Someone in the Knights noticed the direction the bandits had fled, which gave them a general area to look," Edelgard added. A final lie. Hubert had noticed, remembered where Monica was kept, put two and two together, then informed Edelgard, who then told him to mention said direction offhandedly in earshot of Jeralt and Alois.

"...I hope you have more to offer me than excuses, Flame Emperor," Arundel sneered, unable to refute her carefully crafted narrative. There was just too much plausible deniability in it. Still, this was a problem, and the possibility of Edelgard being a clever schemer who planned all this and covered her tracks wasn't something he would ignore. So without saying as much, he was offering her an opportunity to prove her loyalty to determine whether or not he cut her loose.