This reveal was prompted by a tweet from venerable Star Wars nerd archive Wookieepedia, which noted last week that Hamill had provided the voice of Mandalorian bartender droid EV-9D9. Hamill responded by saying that he actually had “multiple secret voice cameos in every Star Wars movie released since 2015"—which covers all of the Disney movies, even the ones Luke Skywalker is not in. Disney and Disney-owned studios have a pattern of selecting voice actors who are a good luck charm and putting them in every movie, be it Alan Tudyk for Disney Animation or John Ratzenberger for Pixar, so it seems like Mark Hamill has become the Lucasfilm version of that (assuming he shows up in other live-action shows or in that new Indiana Jones movie).
So who are these secret characters? Wookieepedia knows about a couple of named aliens in the sequel movies, but those definitely aren’t secret, and the page says nothing about Hamill being in Solo or Rogue One. Obviously, then, somebody needs to sit down and watch all of the Disney-era Star Wars movies, even whichever one or ones they don’t like, and listen close to pick out which characters talk like Mark Hamill, or which characters make little guttural noises like Mark Hamill… then again, his Luke Skywalker voice is completely different from his Joker voice, so there’s a good chance you could hear him and not know it. Hamill offered a hint of sorts a while back, explaining that any Star Wars character played by Patrick Williams is actually him, but it seems like he’s usually just mentioned in the credits without saying who he plays. The actual only solution, then, is to just assume that every voice in Star Wars, unless it’s in the credits as someone else (and not this fake someone else), is secretly Mark Hamill.