mystery convention

We went for a walk Sunday morning and noticed backpacks everywhere. There were so many that we asked what was up and these guys they said they were attending a mystery convention … then they smiled and said there were geeks (smile). I assume their backpacks were full of cards or comic books which they traded. I have googled the subject but have not found the name of the event or a website. But we can attest it was well attended given we saw several hundred people with backpacks headed to the Convention Center.