Since they spoke in hushed voices, it became impossible for him to know exactly what they were talking to each other, but that doesn't mean he couldn't understand the whole situation.
He faintly got an idea about whom they were talking about as he heard them calling the princess name and his ears sure caught few words along the way, allowing him to connect the dots, however, the whole picture isn't clear to him.
So he took a chance and got closer to them, and finally stood behind the wall, to eavesdrop their conversation. However, to his surprise, as soon as he halted his steps and thought he could hear whatever they were talking about, their conversation too halted.
The young maid stopped with an incomplete sentence, following it Shirley turned around reflexively.
