I just noticed this, remember the scene in which Marco find out that Reiner and Bertholdt are titans?
Marco had been behind the building, eavesdropping on Reiner and Bertholdt since the beginning and only revealed himself to ask them what they were talking about AFTER hearing about "my titan" and "the hole we worked so hard to make."
When he asked them what they were talking about, it wasn't out of shock, it was a deliberate choice he made. It is also further evidence that he was being truthful about his desire to talk things out with the warriors.
