I think you mean a dog whistle
dog whistle noundog whistle noun
(TO CALL DOG) a whistle used to train dogs, with a very high sound that humans cannot hear
(POLITICS) (disapproving): a remark, speech, advertisement, etc. by a politician that is intended to be understood by a particular group, especially one with feelings of racism or hatred, without actually expressing these feelings: "Attacks on "political correctness" can be a dog whistle to rally white voters."