If you have kept up with the thousands of screeching goblins who have turned the internet into a weird quicksand pit of "Well, ACTUALLY," you probably guessed that this particular theory is an offshoot of QAnon. If you're not, the idea behind QAnon is that Trump, when he's not ruling America like a drunken fourth-grader in the back of a Denny's, is secretly prosecuting a cabal of traitors and Washington elites that include McCain and the likes of Barack Obama and Hillary Clinton.
What McCain's particular act of treason was supposed to be tends to vary. Either it's complicity in the general cabal or another wildly debunked claim that McCain betrayed his country by being captured, imprisoned, and tortured during the Vietnam War. But the smoking gun here is McCain's use of a medical boot back in November 2017, which he wore after he supposedly injured his Achilles tendon. Those who know the TRUTH know that the boot was obviously meant to hide the tracking bracelet on his ankle.