How did you come to the conclusion that she suspected her knight? The "... you fool." doesnt seem to be enough for that.
Yeah, it's a bit ambiguous. It's odd for her to call him a fool for dying during an operation where he was working as a bodyguard, on a task that is known to lead to deaths nearly every time. Given her personality, I would not expect her to call him a fool for not leaving Deculein to die and saving himself.
But it is also too subdued for her realizing that he tried to murder Deculein after the ambush, and died instead. But maybe Deculein's answer combined with how odd the "second ambush" was, gave her some hints? I mean, what everyone saw was that after the ambushers were defeated and the passengers were safe, the knight returned to the train and a second fight happened, from which only Deculein emerged. The "second ambush" is speculation, but if she had an inkling of how he saw Deculein, maybe she could have suspicions.