Secret Investigation
Undercover Investigation
noun, verbal noun
When you go inside somewhere to quietly spy on someone, you're doing a secret investigation.
内偵 is a formal word for investigations where the target doesn't know they're being investigated. It can mean an undercover investigation by police, or it can be a secret check on a business partner, a new hire, or even a suspicious spouse by companies or detectives.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
警察はどんな時に内偵捜査をするのですか。
When do the police conduct their secret internal investigation?
あのレストラン、税務署の内偵調査が入ったらしいよ。
I heard that restaurant was secretly investigated by the tax office.
探偵事務所に夫の浮気の内偵調査をお願いしました。
I asked a detective agency to secretly investigate my husband's affair.
京都府警が内偵を開始したらしいよ。
I heard the Kyoto Prefectural Police have started a covert investigation.