"The Sacrifice" the new L4D2 addon wherein you play as the L4D1 characters is out: http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802454108d4/?of=3

Which prompts this question from me since I recently played "The Passing". The mechanic in L4D2 makes a comment about having run into the L4D1 characters before, which made me wonder if I had missed something. Is "The Sacrifice" the missing chapter that will explain that little aside? I assume it also explains why one of the L4D1 characters is missing in "The Passing"?

Anyone know?