The outlet has speculated that Tatum flying down to Comic Con just to take one selfie would not make sense. But there are possibilities that he could have flown down from some other location to San Diego. The location maybe could be a place somewhere in Canada.
Bryan Singer directed “X-Men: Apocalypse” has been shooting in Canada for quite some time now. Singer was to fly directly to San Diego from Canada to promote “X-Men: Days of Future Past- Rogue Cut” while the cast that was also flying with him was to be a surprise. The cast members of the movie including Michael Fassbender, James McAvoy, Jennifer Lawrence, Nicholas Hoult, Oscar Isaac, Olivia Munn amongst others surprised everyone with their presence when they joined Singer on stage. Since the cast has also been shooting for the movie in Canada, chances are even they flew in from the same location.
What proved to be a bigger surprise when Tatum joined the cast and others moments before the selfie was taken. Flying in just to take a random selfie seems a bit odd but what if he actually flew in with the cast from Canada?
http://www.movienewsguide.com/x-men-apocalypse-movie-update-channing-tatum-hints-gambit-cameo/74846I do not know. I just think that Tatum wanted to make some hype around Gambit. His shirt is a good example of it.