Use the adjective clandestine to describe something that is done in secret, like your clandestine attempts to steal your brother's Halloween candy.
Clandestine, an adjective imported from Latin, describes a secret, usually illegal activity. Often these things have to do with political and spy organizations. The CIA might run a clandestine operation to infiltrate terrorist organizations. Words with similar meanings include secret; covert, used especially for military matters; undercover, common in espionage; and surreptitious which emphasizes a person's efforts to keep something secret.