What did Jack say is the big question?
Watch the clip again... Jack asks Marshall: "What archive were you logged into?" (or something similar)
Marshall stammers something about it being wrong... but Jack's response is answering his question to Marshall a few seconds earlier... "It's our own."
Marshall was logged into APO archives (which for him is probably standard practice) and BOOM, proof we've got a double agent in APO. The big question shouldn't be what did Jack say. It should be, who is the traitor in APO? And Sloane is such an easy answer, it's not him. I'm with either Agent Grace (as the obvious "not-Sloane" pick) or I'll take Dixon.