undercover

working or done secretly in order to find out information for the police, a government, etc. 秘密工作的；暗中做的；私下进行的
an undercover agent
密探
an undercover operation/investigation
秘密行动／调查

adverb

The illegal payments were discovered by a journalist working undercover.
这些非法付款是一位暗中查访的新闻记者发现的。