I Am Dumb And Need Someone To Explain 'Cache' and Roger Ebert Please

Can someone please explain what Roger Ebert is talking about in his recent piece on Cache please? He wrote: “Now I call your attention to the shot I missed the first time through. You will find it on the DVD, centering around 20:39. You tell me what it means. It’s the smoking gun, but did it shoot anybody?” Nobody seems to understand what he is talking about.