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.