Lan Wangji actually does appear visibly, if faintly, alarmed, at even the suggestion of Wei Ying not having noted his absence. "We were separated during a night-hunt. Wei Ying looked for me all the while that I was gone, and found me thirteen days later." The little trick of timing does not escape him there. He considers mentioning it, even -- but for now he will wait to hear the reason that Nightingale is asking.