We have 4000soldiers and many other tons of character, where are all of them? No single dead body around the field or any blood? I doubt that Kaido and Orochi caught all 4000soldiers without killing them.
Be honest I smell here a trick/trap of Law. His face in chapter 955 was already very suspect. Seeing what happend now,I can imagine that Law and co try to bring fake news to Orochi who gonna believe that shit.
Which means the traitor wasn't know about Law plan, fits to the Situation of the 9 RS. They left days ago before Law came back do the traitor must be one of the 9Rs who fall in the trap ot Law(if the speculation about the trick are true).
If not then we most like getting a flashback next week. But I think all this is just a trick of Luffy/Law.