大眾的用法是quietly. He quietly sits behind her.
高級的字是surreptitiously. A Russian merchant ship whose owner has allegedly carried weapons for the Kremlin turned off its transponder last month before surreptitiously docking at South Africa’s largest naval base. 一艘俄羅斯據信載著武器的商船偷偷地停泊在南非最大的海軍基地
也可用furtively:
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