Word for a message that most people find plain but a smaller group see as supportive

I feel like there's an idiom for this, such as if someone makes a comment that might seem innocuous at face value so you dismiss it, but another group of people who are more extreme might see as a sign that the person is on their side.

Something like "Well, some people, criminals, who are coming into this country don't belong here." Where a racist or a white nationalist might think "He supports our vision of the pure race," but the rest of us would think "Yeah, I don't want criminals coming into our country".

I feel like it's similar to "false flag" or something, but I can't remember quite the right word.

An example sentence, in response to a statement by someone else to call them out on their ____-ing:

What you're doing here is saying "This is Mexico's fault" but that's a ____ for people who don't think Mexicans belong here.

I think you mean a dog whistle

dog 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."

A shibboleth is a word or term that is only used in a certain way by a certain group of people, and it enables these people to recognise a fellow member. Outsiders may not notice anything out of the ordinary, and/or they may be unable to properly use the term.

Some general, tangentially related terms are sign, tell-tale, pejorative, dysphemism, discours.

This is sometimes referred to as coded [X] (where X is usually racism).

It's usually used by opponents of the speaker to imply the presence of the “dog whistle” ...