Undercover (adjective)

1. Working secretly in order to get information or to stop something illegal.
2. Not openly or publicly stated.

Origin:

Early 20th century: from under- + cover.

Examples:

1. She went undercover as a waitress to gather information about the restaurant.
2. The police had an undercover officer in the gang.
3. The company is using an undercover investigator to catch the thief.
4. He was an undercover journalist investigating corruption in the government.
5. The couple's marriage was an undercover love affair that lasted for years.