Some of the more tech-savvy gamers have discovered mention of a possible fourth campaign for Resident Evil 6, starring Ada Wong, hidden away in the recently released demo's files. Posts at NeoGAF and GameFaqs (via Destructoid) pull out the individual lines of code which would seem to confirm the existence of this fourth, so far unannounced campaign.
Assuming the information discovered in the files is accurate, it looks like this fourth campaign brings back Crimson Head zombies and also features a few vehicle-based sections. Ada's co-op partner is only referred to as "HELPER," though some believe that Carla Radames -- Ada's clone, based on leaked RE6 info from earlier in 2012 -- will be the second character.
None of this is confirmed of course, though the evidence remains pretty compelling. It's also entirely possible that this fourth campaign exists on paper and is planned as future DLC. Until Capcom confirms something, it's all just speculation.