I think the "虚" here is the same "虚" as "心虚“. As in having a guilty conscience. So "Xiaolu looked guilty"? But it all depends on context. Has he done anything to make himself feel guilty?
发虚 means to feel timid or apprehensive, to lose confidence. So agreed about 心虚, but it might not be the guitly conscience meaning, it might be the timid meaning, as in 心虚胆怯. But yeah, depends on context.