Celebrities have a long storied history of wandering around SDCC in secret by way of cosplay. It's a good way for them to explore the floor without getting mobbed by everyone in sight, and also makes for good publicity, which I am indulging in now.
While Henry Cavill could probably do with some more skills in the cosplay department, essentially just dawning a Guy Fawkes mask, it did the job while he took to the floor. He also used it to surprise Will Smith and others from the Suicide Squad cast. This is a cute little video.
Take a look: