keeping a secret or furtive watch
the act of keeping a secret watch for intelligence purposes
同义词：undercover work
the act of detecting something; catching sight of something
同义词：detection??/??catching??/??espial??/??spotting
英汉双解词典包含3185865条英汉词条，基本涵盖了全部常用单词的翻译及用法，是英语学习的有利工具。