secret scheme; hidden agenda; trick up one's sleeve
Idiomatic expression meaning to harbor a secret plan or ulterior motive. Often used with verbs like ある or 抱く. Compare 腹に一物, which has the same meaning but uses 腹 (belly) instead of 胸 (chest).
See also: 腹に一物
Behind his kind attitude, he must have a secret scheme.
It seems he harbors a hidden agenda beneath that smile.
The phrase combines 胸 (chest, heart) with 一物 (one thing, a certain matter), literally 'one thing in the chest.' It figuratively refers to a hidden thought or scheme. The exact origin is uncertain, but it follows a common pattern of using body parts to represent inner thoughts.