In order to promote Star Wars Force for Change, Mark Hamill donned a Stormtrooper costume and took to the streets to tell people they could win a trip to the premiere. He dropped a few hints, even outright told someone who he was, and yet no one caught on.
Between this and that time Harrison Ford dropped in on a bunch of Skype calls, we need to be very suspicious of people with camera crews promoting the charity event. Stay vigilant! Who knows where Carrie Fisher might show up!