Edmund knew Lucy. In or out of Narnia, she had always been brave. Courage - valiance - wasn't foolhardiness, the lack of fear when there was very good reason to be afraid; however Lucy still didn't frighten easily. If she were this shaken, there was good reason. "What kind of trouble?" he asked quietly, running through a mental file of possibilities.
His first thought, the one he kept sticking on, wondered if it had something to do with Peter's absence (the word departure suggested more willingness than Edmund could quite believe the case, and he continued to doubt the official story). That didn't make sense, however. Lucy wouldn't ask for him to tell him about something he already knew, not unless she's discovered something so awful about Peter's disappearance that it could only be said in person.