Why would they look at his arm though? He's covered it up, and it's not like they're gonna randomly think, "Oh, let's look at his arm for no reason." They don't see what we as the viewers see. It actually makes sense here for them to be clueless. But on the other hand, with what happens later..... you should know by now JoJo has some weird logic in certain moments for the sake of epic Stand fights. XD