Maybe use personalized url or purl? If the person visits a personalized landing page, you might be able to track the activity in the landing page performance report. I haven't tried that yet. We did used QR codes on a few of our direct mail pieces, and there were ads in there that had the QR code that takes the person to a marketo landing page. But that doesn't necessarily answer the question that asks if the direct mail was sent and is trackable in SFDC nor in Marketo.