The company suspected there was a mole in the finance department leaking confidential information.

mole 的本义是"鼹鼠"，这种动物生活在地下且不易被发现。在比喻用法中，mole 指"潜伏在组织内部为外部传递信息的间谍或内鬼"，就像鼹鼠一样隐蔽地活动。